PDF Archive

Easily share your PDF documents with your contacts, on the Web and Social Networks.

Share a file Manage my documents Convert Recover PDF Search Help Contact



J Environ Sci Heal A 49, 2014, 753 762 .pdf


Original filename: J Environ Sci Heal A 49, 2014, 753-762.pdf
Title: LESA_A_882180_O

This PDF 1.4 document has been generated by dvips(k) 5.95a Copyright 2005 Radical Eye Software / Acrobat Distiller 10.1.7 (Windows), and has been sent on pdf-archive.com on 06/11/2015 at 19:50, from IP address 71.17.x.x. The current document download page has been viewed 527 times.
File size: 164 KB (11 pages).
Privacy: public file




Download original PDF file









Document preview


This article was downloaded by: [The University of British Columbia]
On: 28 March 2014, At: 12:26
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,
37-41 Mortimer Street, London W1T 3JH, UK

Journal of Environmental Science and Health, Part
A: Toxic/Hazardous Substances and Environmental
Engineering
Publication details, including instructions for authors and subscription information:
http://www.tandfonline.com/loi/lesa20

Thermodynamic properties of chloramine formation
and related reactions during water treatment: A
G4MP2, G4, and W1BD theoretical study
a

Sierra Rayne & Kaya Forest
a

b

Chemologica Research , Mortlach , Saskatchewan , Canada

b

Department of Environmental Engineering , Saskatchewan Institute of Applied Science and
Technology , Moose Jaw , Saskatchewan , Canada
Published online: 28 Mar 2014.

To cite this article: Sierra Rayne & Kaya Forest (2014) Thermodynamic properties of chloramine formation and related
reactions during water treatment: A G4MP2, G4, and W1BD theoretical study, Journal of Environmental Science and Health,
Part A: Toxic/Hazardous Substances and Environmental Engineering, 49:7, 753-762
To link to this article: http://dx.doi.org/10.1080/10934529.2014.882180

PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained
in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no
representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the
Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and
are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and
should be independently verified with primary sources of information. Taylor and Francis shall not be liable for
any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever
or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of
the Content.
This article may be used for research, teaching, and private study purposes. Any substantial or systematic
reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any
form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://
www.tandfonline.com/page/terms-and-conditions

Journal of Environmental Science and Health, Part A (2014) 49, 753–762
C Taylor & Francis Group, LLC
Copyright
ISSN: 1093-4529 (Print); 1532-4117 (Online)
DOI: 10.1080/10934529.2014.882180

Thermodynamic properties of chloramine formation
and related reactions during water treatment:
A G4MP2, G4, and W1BD theoretical study
SIERRA RAYNE1 and KAYA FOREST2
1

Chemologica Research, Mortlach, Saskatchewan, Canada
Department of Environmental Engineering, Saskatchewan Institute of Applied Science and Technology, Moose Jaw, Saskatchewan,
Canada

Downloaded by [The University of British Columbia] at 12:26 28 March 2014

2

A high-level gas and aqueous phase theoretical thermodynamic study was conducted on the primary and related chemical reactions
which occur during chloramination for water treatment using the G4MP2, G4, and W1BD composite methods with the SMD, PCM,
and CPCM solvation models. The standard state (298.15 K, 1 atm or 1M) formation of mono-, di-, and tri-chloramines from their
precursors via hypochlorous acid chlorination is substantially exothermic and exergonic in both the gas and aqueous phases. The
excellent agreement between experimental and theoretical values for a range of structural and thermodynamic calculations on a
suite of calibration compounds suggests that the G4MP2, G4, and W1BD calculations meet or exceed criteria for thermochemical
accuracy. The temperature influence on the thermodynamics of chloramine formation is projected to be negligible regardless of phase
between 0 and 100◦ C. Additional thermodynamic calculations were undertaken on associated chloramination reactions involving the
disproportionation of monochloramine, the decomposition of di- and tri-chloramine, and the reactions of trichloramine with ammonia
and dichloramine. The results from these investigations not only provide a better understanding of the reaction thermodynamics,
they also allow for a more rigorous interpretation of proposed chloramination mechanisms.
Keywords: Chloramination, drinking water treatment, thermodynamic properties, theoretical study, quantum chemistry composite
methods.

Introduction
Despite concerns over disinfection byproduct (DBP) formation,[1,2] chlorination is still the dominant method for
disinfecting water and wastewater streams.[3] When aqueous phase ammonia is present, chloramine formation reactions take place:[4]
NH3 + HOCl → NH2 Cl + H2 O
NH2 Cl + HOCl → NHCl2 + H2 O
NHCl2 + HOCl → NCl3 + H2 O

(1)
(2)
(3)

Despite the fact that monochloramine (NH2 Cl) is a 200fold less potent disinfectant than chlorine, it is also less
reactive towards organic matter, resulting in reduced formation of regulated DBPs while still providing residual
Address correspondence to Sierra Rayne, Chemologica Research,
PO Box 74, 318 Rose Street, Mortlach, Saskatchewan, Canada,
S0H 3E0; E-mail: sierra.rayne@live.co.uk
Received October 30, 2013.

disinfecting power. Dichloramine (NHCl2 ) and trichloramine (NCl3 ) are not desirable disinfectants, since their
disinfecting power is even lower than NH2 Cl and objectionable taste and odor problems may result. Although
HOCl exists in a pH dependent equilibrium with its conjugate base, OCl−, previous work has established that
the reactions for chloramine formation dominantly take
place via the molecular species[5] with an activation energy
on the order of 13 to 19 kJ/mol for NH2 Cl formation.
[5,6]

When chloramination is applied, the pH dependent dosing ratio of chlorine to ammonia must be carefully monitored in order to avoid excess chlorine (which can lead
to greater DBP production) or ammonia (resulting in nitrification concerns) in the resulting water supply. At chlorine:ammonia ratios of ≤ 5:1 by weight and in the pH range
of 6.5 to 8.5, NH2 Cl is the dominant chloramine product.
Above chlorine:ammonia ratios of 7.6:1, breakpoint chlorination takes place, yielding nitrogen gas, nitrate, NCl3 ,
and the reappearance of free chlorine. In practice, a chlorine:ammonia ratio ranging from 3:1 to 5:1 is used during
chloramination.[7]

754

Rayne and Forest

Table 1. Comparison between experimental and calculated gas
phase standard state (298.15 K, 1 atm) enthalpies of formation
( f H◦ (g) ) for ammonia (NH3 ), hypochlorous acid (HOCl), and
water (H2 O) at the G4MP2, G4, and W1BD levels of theory.
Compound

Experimental

G4MP2

G4

W1BD

NH3
HOCl
H2 O

−45.9 [52,53]
−74.5 [52]
−241.8 [52,53]

−43.0
−75.0
−240.8

−42.6
−74.3
−240.1

−47.1
−81.7
−244.9

Downloaded by [The University of British Columbia] at 12:26 28 March 2014

Values are in kJ/mol.

Although the kinetics of chloramination have been
widely studied and mechanisms proposed therefrom,[5,8–14]
relatively little appears to be known about the thermodynamics of these important water treatment processes.
Thus, in the current study we undertake the first high
level theoretical thermodynamic investigation on the primary reactions during chloramination, as well as extend
our studies to a number of other related reactions thought
to be important for this process. Not only does such work
provide a better understanding of the reaction thermodynamics, it also allows for a more rigorous interpretation—
and possible correction—of proposed chloramination
mechanisms.

Materials and methods
Gas and aqueous phase calculations were performed at
the G4MP2,[15] G4,[16] and W1BD [17–19] levels of theory
using the Gaussian 09 (G09) software program. Aqueous
phase studies were conducted with the SMD,[20] PCM,[21–24]
and CPCM [25] solvation models. Natural Bond Orbital
(NBO) analyses were performed using NBO v3 [26–31] within
G09. Molecular structures were visualized using Avogadro
v1.0.3 [32] and Gabedit v2.4.3.[33] All gas and aqueous phase
optimized structures were confirmed as true minima by
vibrational analysis at the same level. Gas phase standard state (298.15 K, 1 atm) enthalpies of formation
( f H◦ (g) ) were calculated using the approach described
elsewhere.[34,35]

Table 2. Calculated gas phase standard state (298.15 K, 1 atm)
enthalpies of formation ( f H◦ (g) ) for monochloramine (NH2 Cl),
dichloramine (NHCl2 ), and trichloramine (NCl3 ) at the G4MP2,
G4, and W1BD levels of theory.
Compound
NH2 Cl
NHCl2
NCl3
Values are in kJ/mol.

G4MP2

G4

W1BD

52.0
135.8
219.1

54.4
139.7
223.8

45.6
130.2
217.4

Fig. 1. Structures of NH3 , HOCl, H2 O, NH2 Cl, NHCl2 , and NCl3
showing bond length and angle descriptors under consideration.

Results and discussion
In previous work,[15–19,34–47] the G4MP2, G4, and W1BD
levels of theory have been shown to yield chemically accurate thermodynamic data on a broad range of inorganic and
organic compounds. To confirm the accuracy of these theoretical methods, we also compared our calculated f H◦ (g)
for ammonia (NH3 ), hypochlorous acid (HOCl), and water
(H2 O) to the corresponding experimental values from the
NIST database.[48]
No experimental f H◦ (g) appears to be available in the
literature for the chloramines. As shown in Table 1, excellent agreement was achieved between the experimental
and theoretical f H◦ (g) for NH3 , HOCl, and H2 O using the
three methods, with average deviations of 1.1 (G4MP2), 1.7
(G4), and 3.8 kJ/mol (W1BD), respectively, all within the
bounds of thermochemical accuracy (±4.2 kJ/mol). The
calculated f H◦ (g) for monochloramine (NH2 Cl), dichloramine (NHCl2 ), and trichloramine (NCl3 ) at the G4MP2,
G4, and W1BD levels of theory range from 45.6 to 54.4
kJ/mol, 130.2 to 139.7 kJ/mol, and 217.4 to 223.8 kJ/mol,
respectively (Table 2).
Further benchmarking of the theoretical methods was
obtained by comparing the gas phase standard state geometries against experimental data [49–51] for NH3 , HOCl,
and H2 O (Figure 1 and Table 3). Excellent agreement was
obtained, with bond length deviations all less than 0.017
˚ and averaging only 0.006 A
˚ . Similarly, all bond angle
A
deviations were less than 1.2◦ , with an average of 0.3◦ .
The W1BD method achieved greater geometric accuracy
˚ and average
(average bond length deviation of 0.005 A

bond angle deviation of 0.1 ) than the G4MP2/G4 meth˚ and averods (average bond length deviation of 0.008 A
age bond angle deviation of 0.7◦ ). The G4MP2/G4 methods tend to overestimate bond lengths and to underestimate symmetric H-R-H (R N,O) bond angles when compared to the W1BD method. However, the G4MP2/G4
methods achieved greater accuracy for the H O Cl bond
angle.
For all compounds, only very minor geometrical
changes are expected upon solvation (Table 4). The SMD

755

Chloramine formation and water treatment study
Table 3. Comparison between experimental and calculated gas
phase standard state (298.15 K, 1 atm) bond lengths and angles
for ammonia (NH3 ), hypochlorous acid (HOCl), and water (H2 O)
at the G4MP2, G4, and W1BD levels of theory.

Downloaded by [The University of British Columbia] at 12:26 28 March 2014

Compound
NH3
r(NH)
∠(HNH)
HOCl
r(OH)
r(ClO)
∠(HOCl)
H2 O
r(OH)
∠(HOH)

Experimental

G4MP2/G4a

W1BD

˚
1.012A
106.7◦

˚
1.017A
105.5◦

˚
1.014A
106.5◦

˚
0.964A
˚
1.691A
102.4◦

˚
0.968A
˚
1.708A
102.4◦

˚
0.967A
˚
1.703A
103.0◦

˚
0.958A
104.5◦

˚
0.962A
103.7◦

˚
0.961A
104.5◦

a
The G4MP2 and G4 methods use the same geometry optimization level
of theory, yielding equivalent structural data.

solvation model generally predicts greater solvation induced alterations to the molecular geometry than do the
PCM and CPCM models. The estimated solvation changes
are likely within the error bounds of the methods. Aqueous

solvation is also expected to result in minor intramolecular
charge redistributions (Table 5). Generally, natural charges
on H, N, and O atoms are predicted to increase in absolute
magnitude upon solvation, whereas the absolute charges on
chlorine atoms will remain constant or decline slightly. Of
note, the chlorine atoms in the compounds under consideration carry modest positive natural charges. With increasing chlorination on the chloramines, the positive charge
on the chlorine increases (from +0.01 to 0.06 depending
on theoretical method/solvation model for NH2 Cl up to
+0.15 to +0.17 for NCl3 ). The SMD solvation model predicts greater ionicity (i.e., ionic bond character) in aqueous
solution for each compound than do the PCM and CPCM
models.
The gas and aqueous phase standard state enthalpies
( rxn H◦ ) and free energies ( rxn G◦ ) of reaction for chloramine formation at the G4MP2, G4, and W1BD levels
of theory are given in Table 6. The reactions are highly
exothermic and exergonic, having rxn H◦ (g) / rxn G◦ (g) of
−69 to −71/−68 to −70 kJ/mol for Reaction 1, −79
to −82/−77 to −80 kJ/mol for Reaction 2, and −76 to
−83/−73 to −79 kJ/mol for Reaction 3. As expected,
entropic effects are negligible, since the number of small

Table 4. Comparison between calculated gas (298.15 K, 1 atm) and aqueous phase (298.15 K, 1 M) standard state bond lengths
and angles for water (H2 O), hypochlorous acid (HOCl), ammonia (NH3 ), monochloramine (NH2 Cl), dichloramine (NHCl2 ), and
trichloramine (NCl3 ) at the G4MP2, G4, and W1BD levels of theory using the SMD, PCM, and CPCM solvation models.
G4MP2/G4a
Compound
H2 O
r(OH)
∠(HOH)
HOCl
r(OH)
r(ClO)
∠(HOCl)
NH3
r(NH)
∠(HNH)
NH2 Cl
r(NH)
r(NCl)
∠(HNH)
∠(HNCl)
NHCl2
r(NH)
r(NCl)
∠(HNCl)
∠(ClNCl)
NCl3
r(NCl)
∠(ClNCl)
a

Gas

SMD

PCM/CPCM c

W1BDb Gas

˚
0.962A
103.7◦

0.964
102.9◦

˚
0.963A
103.1◦

˚
0.961A
104.5◦

˚
0.968A
˚
1.708A
102.4◦

˚
0.972A
˚
1.707A
103.2◦

˚
0.970A
˚
1.707A
102.9◦

˚
0.967A
˚
1.703A
103.0◦

˚
1.017A

105.5

˚
1.018A

104.7

˚
1.018A

104.8

˚
1.014A

106.5

˚
1.020A
˚
1.768A
105.1◦
103.1◦

˚
1.021A
˚
1.774A
104.6◦
102.8◦

˚
1.020A
˚
1.772A
104.8◦
103.0◦

˚
1.017A
˚
1.762A
105.9◦
103.7◦

˚
1.023A
˚
1.769A
101.5◦
110.6◦

˚
1.025A
˚
1.770A
102.0◦
110.1◦

˚
1.023A
˚
1.770A
101.8◦
110.4◦

˚
1.019A
˚
1.765A
102.0◦
110.8◦

˚
1.779A
107.8◦

˚
1.779A
107.8◦

˚
1.779A
107.8◦

˚
1.776A
107.9◦

The G4MP2 and G4 methods use the same geometry optimization level of theory, yielding equivalent structural data.
Solvation calculations are not available at the W1BD level of theory.
c
The PCM and CPCM solvation models yield the same optimized geometries.
b

756

Rayne and Forest

Table 5. Comparison between calculated gas (298.15 K, 1 atm) and aqueous phase (298.15 K, 1 M) standard state natural charges
for water (H2 O), hypochlorous acid (HOCl), ammonia (NH3 ), monochloramine (NH2 Cl), dichloramine (NHCl2 ), and trichloramine
(NCl3 ) at the G4MP2, G4, and W1BD levels of theory using the SMD, PCM, and CPCM solvation models.
G4MP2

Downloaded by [The University of British Columbia] at 12:26 28 March 2014

Compound
H2 O
q(H)
q(O)
HOCl
q(H)
q(O)
q(Cl)
NH3
q(H)
q(N)
NH2 Cl
q(H)
q(N)
q(Cl)
NHCl2
q(H)
q(N)
q(Cl)
NCl3
q(N)
q(Cl)

G4

Gas

SMD

PCM/CPCM b

Gas

SMD

PCM/CPCM

W1BDa Gas

+0.46
−0.93

+0.50
−1.00

+0.48
−0.97

+0.46
−0.91

+0.49
−0.99

+0.48
−0.95

+0.46
−0.91

+0.48
−0.69
+0.21

+0.52
−0.72
+0.20

+0.50
−0.71
+0.20

+0.47
−0.68
+0.21

+0.52
−0.72
+0.20

+0.50
−0.70
+0.21

+0.47
−0.67
+0.20

+0.35
−1.05

+0.37
−1.10

+0.36
−1.08

+0.34
−1.03

+0.36
−1.09

+0.36
−1.07

+0.35
−1.05

+0.36
−0.77
+0.04

+0.39
−0.79
+0.01

+0.38
−0.78
+0.02

+0.36
−0.76
+0.05

+0.39
−0.79
+0.01

+0.38
−0.78
+0.03

+0.36
−0.78
+0.06

+0.38
−0.58
+0.10

+0.42
−0.60
+0.09

+0.40
−0.59
+0.10

+0.37
−0.60
+0.11

+0.41
−0.61
+0.10

+0.39
−0.61
+0.11

+0.37
−0.58
+0.10

−0.48
+0.16

−0.49
+0.16

−0.49
+0.16

−0.51
+0.17

−0.52
+0.17

−0.52
+0.17

−0.46
+0.15

a

Solvation calculations are not available at the W1BD level of theory.
The PCM and CPCM solvation models yield the same natural charges.

b

molecules is equivalent for reactants and products and
there are only modest intermolecular atomic transfers.
The PCM and CPCM solvation models give effectively
equivalent thermodynamic results. The highly negative
rxn H◦ (g) / rxn G◦ (g) indicate that these reactions go to effective completion in the gas and aqueous phases, yielding negligible quantities of reactants under equilibrium
conditions.

The exothermicity/exergonicity increases upon solvation for Reactions 1 and 2, whereas Reaction 3 is essentially
isothermic/isogonic upon solvation. Substantially higher
exothermicity/exergonicity is predicted by the SMD model
versus its PCM/CPCM counterparts for both Reactions 1
(−16 vs. −7 kJ/mol) and 2 (−7 vs. −2 kJ/mol). The temperature influence on reaction thermodynamics was also
investigated between 0 and 100◦ C (Table 7). No significant

Table 6. Calculated gas (298.15 K, 1 atm) and aqueous phase (298.15 K, 1 M) standard state enthalpies ( rxn H◦ ) and free energies
( rxn G◦ ) of reaction for chloramine formation (Reactions 1 through 3) at the G4MP2, G4, and W1BD levels of theory.
rxn H◦
Reaction

Level of theory

NH3 +HOCl→NH2 Cl+H2 O (1)

G4MP2
G4
W1BD
G4MP2
G4
W1BD
G4MP2
G4
W1BD

NH2 Cl+HOCl→NHCl2 +H2 O (2)
NHCl2 +HOCl→NCl3 +H2 O (3)

a

rxn G◦

Gas

SMD

PCM

CPCM

Gas

SMD

PCM

CPCM

−70.9
−68.9
−70.5
−82.0
−80.5
−78.6
−82.6
−81.7
−75.9

−86.2
−84.3
n/aa
−88.8
−87.4
n/a
−83.0
−82.1
n/a

−77.6
−75.6
n/a
−83.8
−82.2
n/a
−82.0
−81.0
n/a

−77.6
−75.6
n/a
−83.8
−82.3
n/a
−82.0
−80.9
n/a

−70.1
−68.0
−69.7
−79.9
−78.5
−76.5
−79.4
−78.6
−72.8

−85.5
−83.6
n/a
−86.8
−85.4
n/a
−79.9
−79.0
n/a

−76.8
−74.8
n/a
−81.8
−80.2
n/a
−78.9
−77.9
n/a

−76.9
−74.9
n/a
−81.8
−80.2
n/a
−78.9
−77.8
n/a

Solvation calculations are not available at the W1BD level of theory. Values are in kJ/mol.

757

Chloramine formation and water treatment study

Table 7. Calculated differences in gas (298.15 K, 1 atm) and aqueous phase (298.15 K, 1 M) standard state enthalpies ( rxn H◦ (g) )
and free energies ( rxn G◦ (g) ) of reaction for chloramine formation at the G4MP2, G4, and W1BD levels of theory between 0 and
25◦ C and between 100 and 25◦ C.
NH3 + HOCl→NH2 Cl + H2 O
Level of theory

Gas

G4MP2
G4
W1BD
G4MP2
G4
G4MP2
G4
G4MP2
G4

−0.02
−0.02
−0.02
−0.02
−0.02
−0.02
−0.02
−0.02
−0.02

G4MP2
G4
W1BD
G4MP2
G4
G4MP2
G4
G4MP2
G4

−0.19
−0.19
−0.19
−0.19
−0.19
−0.19
−0.19
−0.19
−0.19

G4MP2
G4
W1BD
G4MP2
G4
G4MP2
G4
G4MP2
G4

−0.34
−0.34
−0.34
−0.34
−0.34
−0.34
−0.34
−0.34
−0.35

SMD
PCM
CPCM

Downloaded by [The University of British Columbia] at 12:26 28 March 2014

rxn H◦ (g),0→25◦ C

Phase

Gas

SMD
PCM
CPCM

Gas

SMD
PCM
CPCM

rxn H◦ (g),100→25◦ C

0.06
0.06
0.06
0.07
0.06
0.07
0.07
0.07
0.07
NH2 Cl+HOCl→NHCl2 +H2 O
0.54
0.54
0.53
0.56
0.55
0.55
0.55
0.55
0.55
NHCl2 +HOCl→NCl3 +H2 O
0.94
0.94
0.95
0.95
0.96
0.96
0.96
0.95
0.95

rxn G◦ (g),0→25◦ C

rxn G◦ (g),100→25◦ C

−0.07
−0.07
−0.07
−0.07
−0.07
−0.07
−0.07
−0.06
−0.06

0.19
0.20
0.19
0.19
0.18
0.18
0.18
0.19
0.19

−0.18
−0.18
−0.18
−0.18
−0.18
−0.18
−0.18
−0.18
−0.18

0.45
0.45
0.45
0.44
0.44
0.45
0.44
0.44
0.45

−0.28
−0.28
−0.28
−0.28
−0.28
−0.28
−0.28
−0.27
−0.27

0.68
0.68
0.67
0.67
0.67
0.66
0.67
0.67
0.67

Values are in kJ/mol.

influence was found, with the reactions being effectively
isothermic/isogonic over this range.
Several other reactions during chloramination have been
proposed that also warrant thermodynamic investigation.
The following disproportionation reaction has been proposed for monochloramine:[11]
NH2 Cl + NH2 Cl → NHCl2 + NH3

(4)

This reaction is thermodynamically favorable in both
the gas (∼−10 kJ/mol) and aqueous (from −1.3 kJ/mol
down to negative several kJ/mol) phases, although a nonnegligible equilibrium is predicted to exist in solution
(Table 8).
The decomposition of NHCl2 has been studied by Hand
and Margerum.[13] The authors found that the decomposition of NHCl2 is autocatalytic and the rate increases
as NCl3 and HOCl are produced. The reaction between
NHCl2 and HOCl (Reaction 5) is general base (B; where
B = HPO4 2−, OCl−, CO3 2−, and OH−) catalyzed, yielding

NCl3 via a nucleophilic attack from the NHCl2 nitrogen
atom on the chlorine atom in HOCl simultaneous with
the general base assisted proton removal from NHCl2 . The
NCl3 produced in Reaction 5 reacts with NHCl2 to form
dinitrogen gas (N2 ), chloride ion (Cl−), and HOCl (Reaction 6).
NHCl2 + HOCl + B → NCl3 + BH+ + OH− (5)
NHCl2 + NCl3 + 3OH− → N2 + 2HOCl
(6)
+ 3Cl− + H2 O
When water is the general base, Reaction 5 is highly endergonic in both the gas (+864 to +876 kJ/mol) and aqueous (+136 to +238 kJ/mol) phases. For HPO4 2− (gas: −358
to −363 kJ/mol; aqueous: −17 to −21 kJ/mol), CO3 2−
(gas: −463 to −473 kJ/mol; aqueous: −39 to −67 kJ/mol),
and OH− (gas: −73 to −79 kJ/mol; aqueous: −78 to −80
kJ/mol), the reactions are exergonic in both the gas and

758

Rayne and Forest

Table 8. Calculated gas (298.15 K, 1 atm) and aqueous phase (298.15 K, 1 M) standard state enthalpies ( rxn H◦ ) and free energies
( rxn G◦ ) of reaction for Reactions 4, 5, and 6 at the G4MP2, G4, and W1BD levels of theory.

Downloaded by [The University of British Columbia] at 12:26 28 March 2014

rxn H◦
Reaction

Level of theory

Gas

SMD

NH2 Cl + NH2 Cl → NHCl2
+ NH3 (4)

G4MP2
G4
W1BD

−11.1
−11.7
−8.1

−2.6
−3.1
n/aa

NHCl2 + HOCl + B → NCl3
+ BH+ + OH− (5)
B = H2 O
G4MP2
G4
W1BD
G4MP2
B = HPO4 2−
G4
W1BD
G4MP2
B = OCl−
G4
W1BD
B = CO3 2−
G4MP2
G4
W1BD
B = OH−
G4MP2
G4
W1BD
NHCl2 + NCl3 + 3OH− →
G4MP2
G4
N2 + 2HOCl + 3Cl− +
W1BD
H2 O (6)

rxn G◦

PCM

CPCM

−6.2
−6.6
n/a

−6.2
−6.6
n/a

857.0
128.9
230.3
856.8
130.4
230.7
869.0
n/a
n/a
−368.5
n/cb
−25.3
−364.6
n/c
−21.7
n/cc
n/a
n/a
55.5 −16.7
5.4
55.0 −16.1
5.3
68.8
n/a
n/a
−481.5 −51.1 −75.1
−475.6 −47.6 −70.8
−471.3
n/a
n/a
−82.6 −83.0 −82.0
−81.7 −82.1 −81.0
−75.9
n/a
n/a
−997.3 −718.1 −887.7
−992.9 −715.8 −883.7
−1032.6
n/a
n/a

230.3
230.8
n/a
−25.5
−21.9
n/a
5.4
5.3
n/a
−75.2
−70.9
n/a
−82.0
−80.9
n/a
−887.5
−883.4
n/a

Gas
−9.9
−10.4
−6.9

SMD
−1.3
−1.8
n/a

PCM

CPCM

−4.9
−5.4
n/a

−4.9
−5.4
n/a

864.3
136.4
237.7
864.1
137.9
238.2
876.3
n/a
n/a
−362.5
n/c
−20.1
−358.5
n/c
−16.5
n/c
n/a
n/a
59.6
−12.6
9.5
59.2
−12.0
9.4
72.9
n/a
n/a
−473.2
−42.7
−66.9
−467.3
−39.2
−62.6
−463.1
n/a
n/a
−79.4
−79.9
−78.9
−78.6
−79.0
−77.9
−72.8
n/a
n/a
−1390.3 −1111.1 −1280.7
−1385.8 −1108.9 −1276.7
−1425.5
n/a
n/a

237.7
238.2
n/a
−20.6
−17.0
n/a
9.5
9.4
n/a
−67.0
−62.7
n/a
−78.9
−77.8
n/a
−1280.5
−1276.4
n/a

a

Solvation calculations are not available at the W1BD level of theory.
Calculation failed to converge absent an imaginary frequency.
c
Calculations at the W1BD level of theory were too computationally expensive for this reaction. Values are in kJ/mol.
b

aqueous phases. With OCl−, the theoretical methods predict an endergonic gas phase reaction (+59 to +73 kJ/mol)
and are unclear on the nature of the corresponding aqueous
phase reaction depending on the solvation model chosen
(−13 kJ/mol [SMD] to +10 [PCM/CPCM] kJ/mol). Reaction 6 is projected to be exergonic in both the gas (−1386
to −1426 kJ/mol) and aqueous (−1109 to −1281 kJ/mol)
phases.
Kumar et al.[10] investigated the decomposition of NCl3
under basic conditions, proposing that this chloramine degraded in aqueous solution via individual Reactions 7 and
8, which give the overall decomposition Reaction 9.
NCl3 + OH− → NHCl2 + OCl−
NCl3 + NHCl2 + 5OH− → N2 + 2OCl−
+ 3Cl− + 3H2 O
2NCl3 + 6OH− → N2 + 3OCl− + 3Cl−
+ 3H2 O

(7)
(8)
(9)

Reaction 8 is fast, while Reaction 7 is thought to
be rate-limiting and include the following individual
steps.
NCl3 + OH− → [Cl2 NClOH]−
(10)
[Cl2 NClOH]− + OH− → NCl−
2

NCl−
2

+ OCl− + H2 O
+ H2 O → NHCl2 + OH−

(11)
(12)

The intermediate [Cl2 NClOH]− was proposed to also
react with the hydronium cation to produce NHCl2 , HOCl,
and H2 O (Reaction 13), the protonated general base (BH+)
to give NHCl2 , HOCl, and B (Reaction 14), and with H2 O
to yield NHCl2 , HOCl, and OH− (Reaction 15).
[Cl2 NClOH]− +
+
[Cl2 NClOH]− +
+
[Cl2 NClOH]− +
+

H3 O+ → NHCl2
HOCl + H2 O
HB+ → NHCl2
HOCl + B
H2 O → NHCl2 + HOCl
OH−

(13)
(14)
(15)

where the general base is water, Reactions 13 and 14 are
equivalent. Thus, Kumar et al.[10] proposed that the basecatalyzed decomposition of NCl3 proceeds via a common
reactive intermediate, [Cl2 NClOH]−, which can react with
buffer acids to protonate the nitrogen atom and produce
HOCl or react with the hydroxide anion to yield OCl−. The
resulting NHCl2 rapidly reacts with an additional NCl3 to
give N2 , OCl−, and Cl−.

759

Downloaded by [The University of British Columbia] at 12:26 28 March 2014

Chloramine formation and water treatment study
At the G4MP2, G4, and W1BD levels of theory, we are
unable to confirm the existence of a discrete [Cl2 NClOH]−
intermediate in Reactions 10, 11, 13, 14, and 15. Calculations using the G4MP2 and G4 methods with the PCM
and CPCM solvation models failed to converge on a stable
geometry for this molecule. Aqueous phase calculations using the SMD solvation model at these two levels of theory
converge on a Cl2 N−···ClOH adduct absent any imaginary
frequencies and with a NNCl2- ···ClHOCl interatomic distance
˚ , which is too long to be a covalent N-Cl bond. For
of 2.15 A
comparison, the N-Cl bond lengths in the NCl2 − species are
˚ . At the G4MP2/G4 levels in the gas phase, we
only 1.80 A
find similar results, having a NNCl2- ···ClHOCl interatomic
˚ versus the N-Cl bond lengths in the
distance of 2.20 A

˚.
NCl2 species of only 1.81 A
However, the gas phase G4MP2/G4 calculations give
a single imaginary frequency, signifying a transition state.
Note that the vibrational spectra of transition states are
characterized by one imaginary frequency. This implies a
negative force constant, whereby the energy has a maximum
in one direction in nuclear configuration space, whereas
the energy is a minimum in all other orthogonal directions.
W1BD calculations on this compound were too computationally expensive and did not converge.
Consequently, it appears that [Cl2 NClOH]− present during the decomposition of NCl3 under basic conditions may
be a transition state during the following reaction:

NCl3 + 2OH− → NCl−
2 + OCl + H2 O

aqueous phases (−9 kJ/mol), but the SMD solvation model
suggests an endergonic reaction (+12 to 13 kJ/mol). Reaction 8 is highly exergonic in both the gas (−1333 to −1389
kJ/mol) and aqueous (−914 to −1129 kJ/mol) phases, as
is Reaction 9 (gas phase: −1392 to −1462 kJ/mol; aqueous
phase: −903 to −1139 kJ/mol). Reaction 12 is estimated
to be energetically unfavorable regardless of phase (gas:
+165 to +175 kJ/mol; aqueous: +35 to + 76 kJ/mol). The
reaction of trichloramine with hydroxide anion (Reaction
16) is predicted to be exergonic in both the gas (−225 to
−248 kJ/mol) and aqueous (−23 to −86 kJ/mol) phases.
Thus, the computational results suggest that the mechanism proposed for Reaction 7 (i.e., Reactions 10 through
12) is unlikely, since Reactions 10 and 11 appear to involve
a transition state species rather than a true intermediate,
and Reaction 12 is predicted to be endergonic.
Yiin and Margerum [14] also studied the reactions of
trichloramine with ammonia and dichloramine. These authors reported that the first step in the reaction between
NCl3 and NH3 involves a base- or acid-assisted transfer
of Cl+ to yield NHCl2 and NH2 Cl (Reaction 17). Subsequently, a rapid reaction occurs between NHCl2 and NCl3
to produce N2 , Cl−, and HOCl (Reaction 18). The HOCl
from Reaction 18 reacts with excess ammonia to give additional NH2 Cl (Reaction 1 discussed above). The overall
stoichiometry is shown in Reaction 19.

(16)

NCl3 + NH3 → NHCl2 + NH2 Cl
NCl3 + NHCl2 + 3OH− → N2 + 2HOCl
+ 3Cl− + H2 O
2NCl3 + 3NH3 + 3OH− → 3NH2 Cl + N2
+ 3Cl− + 3H2 O

The calculated gas and aqueous phase standard state enthalpies and free energies for Reactions 7, 8, 9, 12, and 16
are given in Table 9. Reaction 7 is predicted to be exergonic
in gas ( rxn G◦ = −59 to −73 kJ/mol) and PCM/CPCM

(17)
(18)
(19)

Table 9. Calculated gas (298.15 K, 1 atm) and aqueous phase (298.15 K, 1 M) standard state enthalpies ( rxn H◦ ) and free energies
( rxn G◦ ) of reaction for Reactions 7, 8, 9, 12, and 16 at the G4MP2, G4, and W1BD levels of theory.
rxn H◦
Reaction

Level of theory

NCl3 + OH− → NHCl2
+ OCl− (7)

G4MP2
G4
W1BD
G4MP2
G4
W1BD
G4MP2
G4
W1BD
G4MP2
G4
W1BD
G4MP2
G4
W1BD

NCl3 + NHCl2 + 5OH−
→ N2 + 2OCl−
+3Cl− + 3H2 O (8)
2NCl3 + 6OH− → N2 +
3OCl− + 3Cl− +
3H2 O (9)
NCl2 − + H2 O →
NHCl2 + OH− (12)
NCl3 + 2OH− → NCl2 −
+ OCl− + H2 O (16)
a

rxn G◦

Gas

SMD

PCM

CPCM

Gas

SMD

PCM

CPCM

−55.5
−55.0
−68.8
−1273.4
−1266.4
−1322.2
−1328.9
−1321.4
−1391.0
160.1
158.0
167.7
−215.6
−213.0
−236.6

16.7
16.1
n/aa
−850.7
−848.0
n/a
−834.0
−831.9
n/a
28.4
27.5
n/a
−11.7
−11.4
n/a

−5.4
−5.3
n/a
−1062.5
−1056.2
n/a
−1067.9
−1061.5
n/a
68.8
67.1
n/a
−74.2
−72.4
n/a

−5.4
−5.3
n/a
−1062.3
−1055.9
n/a
−1067.7
−1061.2
n/a
68.7
67.1
n/a
−74.1
−72.4
n/a

−59.6
−59.2
−72.9
−1340.3
−1333.2
−1388.9
−1399.9
−1392.4
−1461.8
167.5
165.4
175.1
−227.1
−224.6
−248.0

12.6
12.0
n/a
−917.6
−914.8
n/a
−904.9
−902.8
n/a
35.6
34.7
n/a
−23.0
−22.7
n/a

−9.5
−9.4
n/a
−1129.4
−1123.0
n/a
−1138.8
−1132.4
n/a
76.0
74.4
n/a
−85.5
−83.8
n/a

−9.5
−9.4
n/a
−1129.1
−1122.8
n/a
−1138.6
−1132.1
n/a
76.0
74.3
n/a
−85.5
−83.7
n/a

Solvation calculations are not available at the W1BD level of theory. Values are in kJ/mol.

760

Rayne and Forest

Table 10. Calculated gas (298.15 K, 1 atm) and aqueous phase (298.15 K, 1 M) standard state enthalpies ( rxn H◦ ) and free energies
( rxn G◦ ) of reaction for Reactions 17 through 22 at the G4MP2, G4, and W1BD levels of theory.

Downloaded by [The University of British Columbia] at 12:26 28 March 2014

rxn H◦
Reaction

Level of theory

NCl3 + NH3 → NHCl2
+ NH2 Cl (17)

G4MP2
G4
W1BD
G4MP2
G4
W1BD
G4MP2
G4
W1BD
G4MP2
G4
W1BD
G4MP2
G4
W1BD
G4MP2
G4
W1BD

NCl3 + NHCl2 + 3OH−
→ N2 + 2HOCl +
3Cl− + H2 O (18)
2NCl3 + 3NH3 + 3OH−
→ 3NH2 Cl + N2 +
3Cl− + 3H2 O (19)
NCl3 + NHCl2 + OH−
→ N2 Cl4 + H2 O +
Cl− (20)
N2 Cl4 + OH− → N2 Cl2
+ HOCl + Cl− (21)
N2 Cl2 + OH− → N2 +
Cl− + HOCl (22)

rxn G◦

Gas

SMD

PCM

CPCM

Gas

SMD

PCM

CPCM

11.7
12.9
5.5
−997.3
−992.9
−1032.6
−1127.3
−1117.7
−1168.1
−275.1
−273.6
n/cb
−289.2
−286.1
n/c
−433.0
−433.1
−447.2

−3.2
−2.2
n/a
−718.1
−715.8
n/a
−893.8
−886.8
n/a
−182.6
−181.9
n/a
−196.3
−193.9
n/a
−339.1
−340.1
n/a

4.4
5.4
n/a
−887.7
−883.7
n/a
−1038.5
−1029.5
n/a
−238.1
−236.6
n/a
−253.7
−250.8
n/a
−395.9
−396.3
n/a

4.4
5.3
n/a
−887.5
−883.4
n/a
−1038.4
−1029.4
n/a
−238.0
−236.5
n/a
−253.7
−250.7
n/a
−395.8
−396.2
n/a

21.1
22.3
14.9
−1062.1
−1057.7
−1097.4
−1157.7
−1148.1
−1198.5
−261.3
−259.8
n/c
−333.7
−330.6
n/c
−467.2
−467.3
−481.3

6.2
7.1
n/a
−783.1
−780.8
n/a
−924.4
−917.4
n/a
−168.8
−168.1
n/a
−240.9
−238.5
n/a
−373.3
−374.3
n/a

13.8
14.8
n/a
−952.6
−948.5
n/a
−1068.9
−1060.0
n/a
−224.3
−222.9
n/a
−298.2
−295.3
n/a
−430.1
−430.4
n/a

13.8
14.7
n/a
−952.3
−948.3
n/a
−1068.8
−1059.9
n/a
−224.2
−222.8
n/a
−298.1
−295.2
n/a
−430.0
−430.3
n/a

a

Solvation calculations are not available at the W1BD level of theory.
Calculations at the W1BD level of theory were too computationally expensive for this reaction. Values are in kJ/mol.

b

Reaction 18 (which is a general base-catalyzed reaction,
whereby the OH−/H2 O reactant/product pair can be replaced by PO4 3−/HPO4 2−, NH3 /NH4 +, etc.) is thought
to proceed by way of a tetrachlorohydrazine (N2 Cl4 ) intermediate (Reaction 20), which subsequently decomposes
via OH− attack to form N2 Cl2 (Reaction 21). The N2 Cl2
formed in Reaction 21 is also subject to hydroxide anion
attack to give the final products N2 , Cl−, and HOCl (Reaction 22).
NCl3 + NHCl2 + OH− → N2 Cl4 + H2 O + Cl−

(20)

N2 Cl4 + OH− → N2 Cl2 + HOCl + Cl−

(21)

N2 Cl2 + OH− → N2 + Cl− + HOCl

(22)

The calculated gas and aqueous phase standard state
enthalpies and free energies for Reactions 17 through 22 are
given in Table 10. Reactions 18 through 22 are all strongly
exothermic and exergonic in both the gas and aqueous
phases, regardless of level of theory or solvation model.
Reaction 17 appears to be modestly endergonic in both
phases.

Conclusion
The gas and aqueous phase standard state thermodynamic properties of the major reactions occurring during
chloramination for water treatment were calculated at the

G4MP2, G4, and W1BD levels of theory employing the
SMD, PCM, and CPCM solvation models. Excellent thermodynamic and structural agreement was found between
experimental and computational data on a suite of calibration compounds, suggesting high accuracy for the theoretical results. The primary reactions for chloramine formation
are substantially exothermic and exergonic in both the gas
and aqueous phases, supporting the experimental kinetic
rate data previously reported in the literature.
Negligible temperature effects on the thermodynamic
properties of these reactions are expected between 0 and
100◦ C. The thermodynamic characteristics for a range
of related reactions believed to occur during chloramination were also calculated. This additional information
will help to better understand the mechanistic pathways
and associated energy changes occurring during the chemical transformations of chloramine application in water
treatment.

Funding
This work was supported by a SIAST Seed Applied Research Program grant to K. Forest, and was made possible by the facilities of the Western Canada Research Grid
(WestGrid: Project 100185 to K. Forest), the Shared Hierarchical Academic Research Computing Network (SHARCNET: Project sn4612 to K. Forest), and Compute/Calcul
Canada.


Related documents


j environ sci heal a 49 2014 753 762
comput theor chem 1031 2014 22 33
j environ sci heal a 49 2014 1228 1235
j mol struct theochem 949 2010 60 69
poster 12aug10
theor chem acct 127 2010 697 709


Related keywords