8 May 1998

Ž .Chemical Physics Letters 287 1998 549–552

Proton transfer reaction of hydrogen chloride with ammonia: is itpossible in the gas phase?

Robert Cazar, Alan Jamka, Fu-Ming Tao )

Department of Chemistry and Biochemistry, California State UniÕersity, Fullerton, Fullerton, CA 92834, USA

Received 12 May 1997; in final form 26 November 1997

Abstract

The mechanism of proton transfer between HCl and NH and the effects of individual water molecules on the reaction3Ž . Žare investigated by calculating the structures and energetics of a series of molecule clusters HCl–NH – H O n s 0, 1,3 2 n

.2, 3 using high level ab initio theory. Without water, the system exists as a simple hydrogen-bonded complex and no protontransfer occurs from HCl to NH . The first water molecule was found to induce a nearly flat potential energy pathway for3

proton transfer, but at least two water molecules must be involved for complete proton transfer from HCl to NH . The study3

supports the likelihood of gas phase proton transfer in HCl–NH in the presence of water vapor. q 1998 Elsevier Science3

B.V. All rights reserved.

Proton transfer plays a key role in a wide range ofchemical and biological reactions. Hydrogen chloride

Ž .and ammonia HCl–NH provide us a prototypical3

acid-base pair for studying proton transfer reactions.In the aqueous solution, the reaction of HCl withNH is instantaneous, and the product is the ion pair,3

NHq PPP Cly, resulting from the transfer of a proton4

from HCl to NH . However, chemists have long3

puzzled over the following questions: what is thedetailed mechanism of proton transfer and what isthe stable form of HCl–NH in the gas phase?3

w xMulliken 1,2 speculated that gas phase HCl–NH 3Ž q y.might exist as an ion pair NH PPP Cl just as in4

the aqueous solution. Early ab initio calculations by

) Corresponding author.

w xClementi 3–5 indeed showed the strong ion-paircharacter for the system. These seem to corroboratewith the well-known observation that a white fog ofsolid ammonium chloride particles appears in theinterdiffusion of the vapors from concentrated am-

w xmonia and hydrochloric acid 6,7 . However, mi-w xcrowave experiments by Legon and coworkers 8,9

conclusively revealed that the system exists as asimple hydrogen-bonded complex with HCl as thehydrogen bond donor and NH as the acceptor rather3

than an ion pair form resulting from proton transfer.This experimental result is supported by several

w xhigher level ab initio calculations 10,11 and byw xmatrix isolation studies 12 .

It seems that water may play a critical role inassisting proton transfer in HCl–NH . In fact, self-3

Ž . w xconsistent reaction field SCRF calculations 11 ofaqueous HCl–NH gave the stable ion pair product3

0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII: S0009-2614 98 00232-2

( )R. Cazar et al.rChemical Physics Letters 287 1998 549–552550

w xas expected. All the SCRF methods 13,14 , how-ever, treated HCl–NH as a solute in a cavity sur-3

rounded by a continuum characterized by the dielec-tric permittivity of liquid water, and therefore nodetails were given about the specific solute-solventinteractions in the framework of the continuummodel. As a result, the exact role played by theindividual water molecules and the detailed mecha-nism of proton transfer in HCl–NH were not delin-3

eated. More specifically, it is not clear whetherproton transfer must take place in the aqueous solu-tion or whether it is possible in the gas phase in thepresence of only a few water molecules. If the latteris true, then what is the minimum number of watermolecules involved in proton transfer in HCl–NH 3

and what is the exact role played by each watermolecule? In this communication, we report ouranswers to these questions from direct calculations

Ž .Fig. 1. Optimized structures of the clusters HCl–NH – H O3 2 nŽ .ns0,1,2,3 . The two numbers shown in each structure are the

˚ Ž . Ž .values in A of the equilibrium distances r H–Cl and r H–Nrespectively.

Fig. 2. Potential energy profiles along the proton transfer pathwayŽ .in HCl–NH – H O . The energy D E is relative to that of the3 2 n

˚Ž .corresponding cluster with r H–Cl s 1.3 A.

of the structures and energetics of the relevantmolecule clusters using high level ab initio theory.

Our ab initio calculations were performed on aseries of four clusters of molecules of the composi-

Ž .tion HCl–NH – H O , where n s 0,1,2,3 and is3 2 n

the number of water molecules. The equilibriumgeometries of these clusters were obtained by fullgeometry optimization at the level of the frozen coresecond-order Møller-Plesset perturbation theoryŽ . w x Ž .MP2 15,16 with the 6-311qqG d,p basis setw x17–19 . The initial geometry of each cluster at thestart of optimization was a hydrogen bonded HCl–NH complex surrounded by zero to four water3

molecules. The potential energy surface along theproton transfer pathway in each cluster was alsoexplored at the same level by a constrained optimiza-tion procedure in which the geometry is re-optimizedwith the only constraint that the reaction coordinate,Ž .r H–Cl , is fixed at a given value.

The equilibrium geometries of the clusters arepresented in Fig. 1. The potential energy curvesalong the proton transfer pathway are presented inFig. 2 where all the energies are relative to that for

˚Ž .r H–Cl s 1.300 A. The first cluster of the series,HCl–NH , contains no water, and the optimized3

Ž .geometry shown in Fig. 1 a is a hydrogen bondedcomplex with C symmetry. This is consistent with3Õ

w xthe microwave experiments 8,9 and previous abw xinitio calculations 10,11 . The equilibrium Cl–N

˚Ž .distance, r Cl–N s 3.132 A, is in good agreement˚ w xwith the experimental value of 3.136 A 8,9 . The

( )R. Cazar et al.rChemical Physics Letters 287 1998 549–552 551

intermolecular dissociation energy D s 9.26e

kcalrmol is large, indicating a very strong hydrogenbond interaction between HCl and NH . The hydro-3

˚Ž .gen bond distance is r H–N s 1.820 A and the˚Ž .H–Cl valence bond distance is r H–Cl s 1.312 A,

˚which is 0.039 A larger than that of isolated HCl. AsŽ .shown in Fig. 2, if r H–Cl is forced to increase

along the C axis, the energy of HCl–NH in-3Õ 3˚Ž .creases monotonically. At r H–Cl s 1.800 A,

which approximately corresponds to an ion-pair formwith the proton transferred from HCl to NH , the3

energy of HCl–NH is 10.65 kcalrmol above that of3

the hydrogen-bonded equilibrium structure. This in-dicates that proton transfer is highly unlikely in pureHCl–NH .3

When a water molecule is brought to HCl–NH ,3Ž .as shown in Fig. 1 b , the water molecule takes a

position which forms an approximately equilateraltriangle for the three molecules in the cluster. Theorientation of the water molecule is consistent withthe electrostatic argument. The water molecule causessome structural changes in HCl–NH ; the C sym-3 3Õ

metry of HCl–NH is broken while the H–Cl bond3˚Ž .distance increases to r H–Cl s 1.361 A and theŽ .hydrogen bond distance decreases to r H–N s

˚1.612 A. The overall effect of the water molecule onthe HCl–NH structure is not significant and HCl–3

NH remains hydrogen-bonded. However, the poten-3

tial energy surface along the proton transfer pathway,as shown in Fig. 2, is much flatter than that in pure

˚Ž .HCl–NH . At r H–Cl s 1.800 A, for example,3

the energy of HCl–NH -H O is only 1.18 kcalrmol3 2

above that of the equilibrium structure. This corre-sponds to 9.47 kcalrmol of stabilization energy forthe ion-pair form due to the water molecule. Thus,one water molecule has a major impact on theenergetics of proton transfer in HCl–NH . Although3

one water molecule is not enough to lead to protontransfer, it results in a much flatter pathway forproton transfer in HCl–NH .3

When a second water molecule is brought into thesystem, some dramatic changes in geometry occur inthe HCl–NH unit. The second water molecule takes3

another triangular position with HCl and NH and is3

symmetric with the first water about the symmetryplane containing Cl–H–N axis. The equilibrium

˚ Ž .H–Cl distance increases from 1.361 A to r H–Cl˚s 1.861 A while the H–N distance, corresponding

to the hydrogen bond distance in the smaller clusters,˚ ˚Ž .decreases from 1.612 A to r H–N s 1.087 A.

Such an equilibrium structure indicates that the H–Clvalence bond is no longer in existence and a newH–N valence bond has formed. As a result, protontransfer from HCl to NH is complete in the pres-3

ence of two water molecules. The proton transfer inthis case may be easily understood by examining thepotential energy curves in Fig. 2. The top three

Ž .curves at a given r H–Cl are nearly equally spaced,indicating that the stabilization energy for the ion-pairform due to the second water molecule is nearlyequal to that due to the first water molecule. Thestabilization energy due to the first water moleculealtered the stability of the hydrogen-bonded form bya large amount, but not quite enough to cause aproton transfer to occur. However, the additionalstabilization energy from the second water moleculecompletely changes the relative stability between thetwo forms and stabilizes the ion-pair form.

When a third water molecule is brought into thesystem, slight changes in the geometry and a signifi-cant change in stabilization occur. The three watermolecules are so positioned that the C symmetry in3Õ

pure HCl–NH is restored. The effect of the third3

water molecule is to move the proton closer to the˚Ž .nitrogen. The r H–Cl increases by 0.083 A to 1.944

˚ ˚Ž .A whereas r H–N decreases by 0.027 A to 1.060A. Nevertheless, the geometry of the HCl–NH unit3

is similar when two or third water molecules arepresent. No major structural changes for the HCl–NH unit are expected when more than three water3

molecules are brought into the system. The potentialenergy curves in Fig. 2 shows a significant stabiliza-tion energy for the ion-pair form due to the thirdwater. Both of the structure and the energetics appearto converge to the aqueous state as the amount ofwater increases to the liquid state.

In summary, we have shown the clear transitionof HCl–NH from a simple hydrogen-bonded com-3

plex to an ion-pair form as an increasing number ofwater molecules are brought into the system. Wehave found that the first water molecule induces anearly flat potential energy pathway for proton trans-fer while the second water molecule allows for aproton transfer from HCl to NH . Since dihydration3

of the HCl–NH complex is not a difficult condition3

to achieve in the gas phase, our results provide

( )R. Cazar et al.rChemical Physics Letters 287 1998 549–552552

strong support for the likelihood of gas phase protontransfer in HCl–NH in the presence of water vapor.3

The presence or absence of water is the key tounderstanding both the microwave experiment of

w xLegon and coworkers 8,9 and the interdiffusionw xexperiment with the HCl and NH vapors 6,7 . The3

microwave experiment was done with the vaporfrom dry ammonium chloride crystals while the in-terdiffusion experiment was done with the vaporsfrom aqueous HCl and NH solutions. In the former,3

only pure HCl–NH was involved and so a simple3

hydrogen bonded structure was found. In the latter,water vapor was unavoidable and so the ionic ammo-nium chloride particles were produced.

Acknowledgements

This work was supported by American ChemicalŽSociety Petroleum Research Fund grant a30399-

. ŽGB6 , the Research Corporation Cottrell College.Science Award , and the Department of Chemistry

and Biochemistry, California State University,Fullerton. We thank Dr. Gene Hiegel for reading andcommenting on the manuscript.

References

w x Ž .1 R.S. Mulliken, J. Phys. Chem. 56 1952 801.w x Ž .2 R.S. Mulliken, Science 157 1967 13.w x Ž .3 E. Clementi, J. Chem. Phys. 46 1967 3851.w x Ž .4 E. Clementi, J. Chem. Phys. 47 1967 2323.w x Ž .5 E. Clementi, J.N. Gayles, J. Chem. Phys. 47 1967 3837.w x Ž .6 E.A. Mason, B. Kronstadt, J. Chem. Educ. 44 1967 740.w x7 B.Z. Shakhashiri, Chemical Demonstrations, 1985, Univer-

sity of Wisconsin, vol. 2, p. 59.w x8 E.J. Goodwin, N.W. Howard, A.C. Legon, Chem. Phys. Lett.

Ž .131 1986 319.w x Ž .9 N.W. Howard, A.C. Legon, J. Chem. Phys. 88 1988 4694.

w x10 Z. Latajka, S. Sakai, K. Morokuma, H. Ratajczak, Chem.Ž .Phys. Lett. 110 1984 464.

w x11 C. Chipot, D. Rinaldi, J.-L. Rivail, Chem. Phys. Lett. 191Ž .1992 287.

w x12 A.J. Barnes, T.R. Beech, Z. Mielke, J. Chem. Soc., FaradayŽ .Trans. 80 1984 455.

w x Ž .13 S. Miertus, E. Scrocco, J. Tomasi, Chem. Phys. 55 1981117.

w x Ž .14 S. Miertus, J. Tomasi, Chem. Phys. 65 1982 239.w x Ž .15 C. Møller, M.S. Plesset, Phys. Rev. 46 1934 618.w x Ž .16 J.S. Binkley, J.A. Pople, Int. J. Quant. Chem. 9 1975 229.w x Ž .17 A.D. McLean, G.S. Chandler, J. Chem. Phys. 72 1980

5639.w x18 R. Krishnan, J.S. Binkley, R. Seeger, J.A. Pople, J. Chem.

Ž .Phys. 72 1980 650.w x19 T. Clark, J. Chandrasekhar, G.W. Spitznagel, P. v. R.

Ž .Schleyer, J. Comp. Chem. 4 1983 294.