ELSEVIER

26 September ! 997

Chemical Physics Letters 276 (1997) 339-345

CHEMIGAL PHYSIC@ LETTERS

Proton transfer reactions within the NH 3 - C H 3OH + cluster

Yue Li *, Xianghong Liu, Xiuyan Wang, Nanquan Lou State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023,

P.R. China

Received 10 June 1997; in final form 23 July 1997

Abstract

The hydrogen-bonding cluster NH3-CH3OH was studied by ab initio calculations at the HF/6-31G * and MP2/6-3G * * levels. Equilibrium geometries of both neutral and ionic NH3-CHaOH clusters, and dissociation channels and dissociation energies of the ionic clusters, are presented. The results show that when NHa-CH3OH is vertically ionized, CH30 and NH~- are the dominant products via a fast proton transfer reaction. For another channel corresponding to the production of CH2OH and NH~', a high energy barrier makes it disfavored. © 1997 Elsevier Science B.V.

1. Introduction

Proton transfer through hydrogen bonding is an important mechanism by which many chemical and biological processes are carried out [1-3]. It has long drawn the attention of both theoreticians and experi- mentalists because of its importance in elucidating various phenomena in nature [4,5].

In multiphoton ionization studies of ammonia, methanol, etc., hydrogen bonding clusters [6-13], the mass spectra exhibited a major sequence of the protonated cluster ions (NH3),H +, (CH3OH),H +, which were produced by the ionization of neutral clusters followed by intracluster proton transfer reac- tions, whereas nonprotonated product ions were few. Tomoda and Kimura suggested that the ammonia dimer cation had two significant channels into NH~ + NH 2 and NH 3 + NH 7, in which the former, cor- responding to a protonation channel, was favored because of its lower dissociation energy [14]. Lee et

* Corresponding author.

al. have investigated protonation reactions occurring within methanol cluster ions. They suggested that CHaOH~-+ CH2OH (and OCH 3) were the products of proton transfer reactions from ionized methanol dimer [6], in which CH2OH came from an intraclus- ter rearrangement reaction. Xia and Garvey have investigated the metastable decomposition of ammo- nia-alcohol mixed cluster ions of the form (ROH)n(NI-I3)m H+, where R = CH 3, C2H 5, C3H 7 with multiphoton ionization [15]. Their results showed that the proton in these mixed cluster ions is bound to an ammonia molecule, forming an NH + core ion.

Apparently little theoretical work has been done on ammonia-methanol mixed clusters to date. In this Letter, we present ab initio calculation results of ammonia-methanol neutral and ionic clusters ob- tained using the GAUSSIA~-94W package. The purpose of this Letter is to investigate intracluster proton transfer and rearrangement reactions within NH 3- CH3OH after it is ionized. The theoretical results can direct further experimental work.

0009-2614/97/$17.00 © 1997 Elsevier Science B.V. All fights reserved. PII S0009-2614(97)00847-6

340 Y. Li et ai. / Chemical Physics Letters 276 (1997) 339-345

2. Computational details 3. Results and discussion

The NH3-CH3OH system was studied by ab initio calculations with the CAUSSlAN-94W program [16]. For all the molecules and complexes considered in this work, the initial geometries were fully opti- mized at the Hartree-Fock level with the 6-31G * standard basis set and further refined with Meller- Plesset perturbation theory and the 6-31G" * basis set. Stationary points were confn'med through the calculation of vibrational frequencies, which were also used to evaluate the zero-point vibrational ener- gies. The frequencies and zero-point vibrational en- ergies were scaled by 0.893 (HF) [17] and 0.93 (MP2) [18] for anharmonicity correction. Since < S 2 > = 0.75-0.77, close to the ideal value 0.75, spin contamination was negligible.

3.1. Equilibrium structures of ammonia-methanol neutral and ionic clusters

The calculated equilibrium geometries of NH 3- CH3OH and its cations, both in their ground states, are shown in Fig. 1, and their geometry parameters are listed in Table 1.

As can be seen from Fig. 1, neutral NH3-CH3OH cluster forms a linear hydrogen bonding structure N . . . H-O, and the distance between N and O is

o

2.9199 A, which is similar to that of other N . • • H-O hydrogen bonding systems [19,20]. The C - O - N atoms have an obvious bend and the O-H bond is a little longer than that in C H 3 O H molecule [R(O-H)

I

N 2 N I

2 4 3 6 5 2

$

5

7 4

(a) (b) (e)

I

N 2

$ O 4 3 2

3 ?

7

(d) (e)

Fig. 1. The equilibrium geometries of the neutral and ionic ammonia-methanol clusters calculated at the MP2/6-31G * * level: (a) neutral cluster; (b) ionic cluster (type I); (c) ionic cluster (type IT); (d) ionic cluster (type In); and (e) "IS. The labels are used to define the geometrical parameters listed in Table 1Table 3.

Y. Li et al. / Chemical Physics Letters 276 (1997) 339-345 341

of CH3OH calculated at the MP2/6-31G * * level is 0.9631 A].

For the ionic NI-I3-CH3OH cluster, geometry optimization gives three equilibrium structures: types I, II and III, which are shown in Fig. lb, c and d, respectively. The bridging H atom of types I and II corresponds to H of the hydroxyl group and ammo- nia, respectively. Type III is the result of an intra- cluster reaction in which H in the methyl group of CH3OH rearranges onto the O atom. From the ener- gies in Table 2, it can be seen that type III is the stablest configuration, which is analogous to the result reported in Ref. [6].

Fig. 1 shows that the three kinds of ionic cluster configurations have nearly linear hydrogen bonding structure. However, being different from the neutral,

the N - O bond length is shortened and the bridging H atom is closer to N. For type I, the hydrogen bonding has an obvious bend, which makes the ionic cluster look like (NH 4 . . . OCH3) + but not (NH 3 • . • HOCH3) +. This kind of change of structure is comparable to that of (H20)~-, (NH3) ~ and (CH3OH) ~- [21].

3.2. Dissociation and proton transfer reactions within (NHs-CH3OH) + clusters

A schematic energy diagram of the intracluster dissociation and proton transfer reactions of ionic ammonia-methanol clusters is shown in Fig. 2, in which (NH3-CH3OH)+er indicates vertically ionized NH3-CH3OH from the optimized ground state of neutral NH3-CH3OH cluster and is set to be zero.

Table 1 Optimized geometrical parameters for the neutral and ionic ammonia-methanol clusters calculated at the MF216-31G * * level a

Neutral Ionic (I) Ionic (11) Ionic (III) TS

Bond lengths (,g,):

H1-N 1.0128 H I - N 1.0207 H I - N 1.0181 H1-N 1.0203 H I - N 1.0198 H2-N 1.0129 H2-N 1.0201 H2-N 1.0184 H2-N 1.0202 H2-N 1.0196 H3-N 1.0128 H3-N 1.0202 H3-N 1.1278 H3-N 1.0201 H3-N 1.0203 H4-N 1.9448 H4-N 1.0524 O-H3 1.3917 H4-N 1.0597 H4-N 1.0607 O-H4 0.9751 O-H4 1.7009 C-O 1.4555 O-H4 1.637 O-H4 1.6257 C - O 1.4113 C-O 1.396 H4-O 0.9668 C-O 1.3971 C-O 1.3989 I-IS-C 1.0882 H5-C 1.0912 HS-C 1.084 HS-O 0.9669 H5-C 1.2463 H6-C 1.0959 H6-C 1.096 H6-C 1.085 H6-C 1.0765 H6-C 1.0821 H7-C 1.0959 H7-C 1.0876 H7-C 1.0864 H7-C 1.0771 H7-C 1.0827

Bond angles (°):

H1-N-H2 106.5862 HI-N-I-I2 108.9622 H I - N - H 2 115.0224 H I - N - H 2 108.993 H1-N-H2 109.0367 H1-N-H3 106.7208 H I - N - H 3 108.9489 HI-N-I-I3 123.1704 H I - N - H 3 108.7536 H1-N-H3 108.7625 H2-N-H3 106.5851 I-I2-N-I-I3 109.0506 H2-N-H3 121.8055 H2-N-I-I3 108.7999 H2-N-H3 108.9795 H1-N-H4 114.4994 H I - N - H 4 109.0422 N - H 3 - O 175.5418 H I - N - H 4 109.8217 H I - N - H 4 110.1351 N - H 4 - O 178.0178 N - H 4 - O 168.4005 H 3 - O - C 123.148 N - H 4 - O 176.43 N - H 4 - O 166.054 H 4 - O - C 106.5644 H 4 - O - C 134.2867 H3-O-H4 120.0526 H 4 - O - C 119.7551 H 4 - O - C 133.496 O - C - H 5 107.5865 O -C -H 5 108.3534 O -C- H5 106.2379 H4-O-H5 122.3172 O- C- H5 54.4302 O - C - H 6 112.8156 O - C - H 6 105.3855 O - C - H 6 109.0295 O - C - H 6 112.5735 O - C - H 6 116.2771 O - C - H 7 112.816 O - C - H 7 113.4769 O - C - H 7 110.4573 O- C- H7 120.7438 O - C - H 7 118.3731

Dihedral angles (°):

C - O - N - H I 61.6975 C - O - N - H 2 - 179.8377 C - O - N - H 3 - 61.4399 H 5 - C - O - H 4 179.9808 H 6 - C - O - H 4 61.1787 H 7 - C - O - H 4 - 61.2164

C - O - N - H 1 - 179.292 C - O - N - H 2 - 62.0556 C - O - N - H 3 63.7298 H5-C-O-H4-- 134.1074 H 6 - C - O - H 4 112.1061 H 7 - C - O - H 4 - 8.9587

C - O - N - H 1 73.1003 C - O - N - H 2 - 106.9556 H4-O-N-H1 - 77.5927 I-I5-C-O-H3 31.9613 H 6 - C - O - H 3 150.0856 H 7 - C - O - H 3 - 87.7653

H 5 - O - N - H I 129.3287 O - H 4 - N - H I 93.1229 H5-C-O-H2-111.7188 O - H 4 - N - H 2 - 146.0007 H 5 - O - N - H 3 9.6963 C - O - H 4 - N - ! 51.1323 C - P - N - H I - 84.9664 H5-C-O-H4 , - 111.061 H6-C-O-I - IS- 167.6068 H 6 - C - O - H 4 148.4091 H 7 - C - O - H 5 - 22.1808 H 7 - C - O - H 4 - 6.4259

a See Fig. ! for pertinent atomic labels.

342 Y. Li et al. / Chemical Physics Letters 276 (1997) 339-345

Fig. 2 implies the results shown as follows, in which D O is the dissociation energy with zero-point energy correction and IPve r is the vertical ionization poten- tial:

NH3-CH3OH ~ CH3OH + NH 3,

D O = 7.02 kcal mol- 1 (1 )

NH3-CH3OH ~ (NH3-CH3OH) + (I) ,

IP = 191.38 kcal mol -I (2)

NH3-CH3OH --=, (NH3-CH3OH)v;r ,

IPve r = 221.61 kcal mol-~ (3)

NH3-CH3OH + (I) ---, CH3OH + NH~,

D o = 37.00 kcal tool-1

CH30 + NH~-, D o = 17.87 kcal tool -1

(4)

(5) NH3-CH3OH + (II) ~ CH3OH + NH~-,

D O = 29.79 kcal mol-I (6)

---, CH3OH ~- + NH 2, D O = 36.32 kcal mol - l

(7) NH3-CH3OH +(III) ---, CH2OH + NH~-,

D O = 19.64 kcal mol -I (8)

(NH3-CH3OH)+er has a higher energy than that necessary for the dissociation to produce NH~+

CH30 or NH~-+ CH2OH. Thus it can be expected that for NHa-CH3OH or larger clusters, the mea- sured ions after ionization should be a sequence of protonated products, in which the proton links pri- marily to N and forms NH~-. The results are logical due to the higher proton affinity of NH 3 than CH3OH and are also in good agreement with the experimen- tal results of Ref. [15]. The channel corresponding to produce CH2OH + NH~ seems to need less energy, but the dissociation process should follow an intra- cluster rearrangement reaction (type I ~ type III), i.e. H in the methyl group moves onto the O atom. At the MP2/6-31G * * level, the calculated confor- mation of the transition state (TS) of the transforma- tion is shown in Fig. le. As shown in Fig. 2, an energy barrier of ~ 30 kcal mol-1 makes the pro- cess less favored than that producing CH30 + NH~-. This conclusion is consistent with experimental re- suits that show that both D+(CD3OH)n and H+(CD3OH)n are formed in the ionization of CD3OH clusters; however, D+(CD3OH)n has less abundance than H+(CD3OH)n [7].

An analysis of the molecular orbitals (MO) shows that the highest occupied molecular orbital (HOMO) of the neutral cluster is located primarily on the O atom; namely, the removal of an electron occurs from the O atom. This is confirmed by a comparison of the atomic charges from the Mulliken population

Tab le 2

Ca lcu la t ed total ene rg i e s E (har t rees) and re la t ive ene rg ie s A E (kcal m o l - 1 ) for the a m m o n i a , me thano l m o n o m e r and c lus te r species

Spec ies E (I-IF) a E ( M P 2 ) b A E (I-IF) A E ( M P 2 )

N H 2 - 55 .557703 - 55 .709963

NI-I~ - 55 .873236 - 56 .028814

NI-I~ - 56 .530771 - 56 .733680

C H 3 0 - 114 .420749 - 114 .709904

C H 2 O H - 114 .408764 - 114 .724234

C H 3 O H - 115 .035418 - 115 .382009

C H 3 O H ~ - 115.338993 - 115 .689434

N I - I 3 - C H 3 O H - 171 .230607 - 171.779022

( N H 3 - C H 3 O H ) ( I ) - 170.977411 - 171 .474059

( N H 3 - C H 3 O H ) (l'I) - 170 .949902 - 171 .459550

( N I - I 3 - C H 3 O H ) ( m ) - 170 .969367 - 171.491145

T S ( I ~ m ) - 170.890619 - 171.423443

( N H 3 - C H 3 O H ) v+r c - 170 .915095 - 171 .422352

- 197 .911150 - 223 .728391

- 39 .088957 - 32 .434250

- 21 .833387 - 23 .333189

- 3 4 . 0 4 3 1 9 7 - 4 3 . 1 5 1 7 8 5

15 .35306 ! - 0 . 684477

0 .0 0 .0

a H F / 6 - 3 1 G * / / I - I F I 6 - 3 1 G * va lues . b M P 2 / 6 - 3 1 G * " / / M P 2 / 6 - 3 1 G * * va lues .

c T h e subscr ip t ve r indicates ver t ical ionizat ion f r o m the op t im ized g r o u n d state o f the neutra l N H 3 - C H 3 O H cluster .

Y. Li et al. / Chemical Physics Letters 276 (1997) 339-345 343

$3

- 0 o

~-100 e~

• ~- -150

-2flO

-2X)

T340en (-23.~

rs(-t~

T~el (-3023) Type m

C-40.~

a-~ot~'+~t~ (1~30)

c~o+~" ( - t~ a-~m+l,K" (-21~

N~-~CH(-221.61)

cKcrI+~ (-214..~9)

l nioaPaemy Fig. 2. Schematic energy diagram for the intracluster dissociation and proton transfer reactions of ionic ammonia-methanol clusters. The energies of all species were obtained at the MP2/6-3 I G * * / / M P 2 / 6 - 3 I G * * level with zero-point vibrational energy corrections. The relative energies in parentheses are relative to the energy of (NI-I3-CH3OH)+er which is set to be zero.

analysis of the neutral and vertically ionized clusters (as shown in Table 3). On vertical ionization associ- ated with type I, the atomic charge of O increases from -0.694 to -0.106 whereas the other atoms change little. The result is unexpected since it is known that methanol has a higher ionization poten- tial (IP) than ammonia [IP(NH3)--10.15 eV, IP(CH3OH) = 10.85 eV]. The IP value calculated in this work of ~ 191.38 kcal mo1-1 (8.30 eV) is

obviously smaller than IP(NH 3) and IP(CH3OH). The formation of a cluster may cause the IP value of a monomer to be red-shifted [22]. However, it is also estimated that more accurate model chemistries and larger basis sets are necessary to predict the energies of the system.

Calculation results indicate that for process (4), when the bond of N-H in hydrogen bonding is considerably elongated, the charges of the ionic clus-

Table 3 Calculated atomic charges of ammonia-methanol mixed clusters

NH3-CH3OH Type I Type II Type I11 TS (NI-I3-CH3OH)v+er H3N. . . HOCH~ a

N - 0.826 - 0.711 - 0.457 - 0.724 - 0.716 - 0.884 - 0,532 O -0,694 -0.378 -0.733 -0.679 -0.584 -0,106 -0.750 C 0.0 -0.085 -0,063 0.396 -0.055 -0.196 -0.175 H 1 0.283 0.400 0.395 0,394 0.392 0.326 0.511 H2 0.287 9.396 0.397 0.393 0.391 0.332 0.511 H3 0.283 0,396 0.549 0.393 0.397 0.326 0.509 H4 0.408 0.456 0.400 0.478 0.468 0.459 0.444 H5 0.109 0.190 0.162 0.017 0.327 0.245 0.158 H6 0.075 0.191 0.184 0.156 0,204 0.249 0.165 H7 0.075 0.146 0.166 0.176 0.176 0.249 0.158

Calculated at the UHFI6-316* level and r (N-H) of N • • • HO structure was fixed at 7.0 ,~.

344 Y. Li et al. / Chemical Physics Letters 276 (1997) 339-345

ter locate dominantly at the N atom and the three hydrogen atoms that are adjacent to the N atom (at the UHF/6-31G* level, when r(N-H) is equal to 7.0 A, the sum of their atomic charges is nearly + 1.0 and the cluster can be considered to have dissociated). Therefore, for (NH3-CH3OH)v+r, the following intracluster charge transfer process may take place accompanying the dissociation of the ionic cluster:

[ H3N " . " (HOCH3) + ]vet

--* [ H 3 N . . . HOCH3] + -~ H3N++ HOCH 3 .

(9)

In order to characterize in more detail the proton transfer potential in the ionic (NH3-CH3OH) + clus- ter (type I), the bridging hydrogen was moved across toward the oxygen in steps. A series of r(N-H) distances were chosen, and for each, the remainder of the geometry of the cluster remained constant according to the conformation of the neutral cluster and the distance of N-O was fixed to be 3.0147 (HF) and 2.9199 A (MP2). Since after vertical ion- ization, relative to the C, N, O heavy atoms, the movement of the lighter H atom should be much faster; the heavy atoms could be considered to be stationary. For the other H atoms in methyl and

lO0

80

60

o m

~g

20

0

-20

/ • HP2

U H

1 [ I I , I , I I I

0.8 1 1.2 1.4 1.6 1.8 2 2.2

r (N-H) (~)

Fig. 3. Proton transfer energy curves of ionic cluster (H3N. • • HOCH3) + (type I) at different calculation levels. Dot- ted curve: the energy curve of the neutral cluster calculated at the RHF/6-31G* level. All the curves illustrate the relative energy, with zero taken at their lowest point.

NH 3, their effects on the bridging hydrogen were neglected• From the calculated conformation, it can be seen that these atoms of the ionic clusters vary little relative to the neutral; therefore, this approxi- marion is reasonable. The obtained energy curve is shown in Fig. 3. It shows that for the neutral cluster (as shown in the dotted line), the bridging H atom tends to be closer to O than N. However, the ionic cluster (type I) is different and two potential wells exist at the UHF/6-31G* level, which correspond to (H3N. -. HOCH3) + and (H4N. • • OCH3) +. The transfer of the proton across to the nitrogen is fa- vored by ~ 30 kcal moi-l . This transfer is blocked by a small harrier of only ~ 6 kcal mo1-1. At the MP2/6-31G * * level, the minimum associated with ( H 3 N . . . HOCH3) + disappears. Thus it is expected that the intracluster proton transfer process from ( H 3 N . . . HOCH3) + to ( H 4 N . . . OCH3) + should occur quickly, and when NH3-CH3OH is ionized, if surplus energy exists to make the ionic cluster disso- ciate, CH30 and NH~ should be dominant products.

4. Summary

Both neutral and ionic NH 3-CH 3OH clusters form a linear hydrogen bonding structure. When NH 3- CH3OH is vertically ionized, the dissociation chan- nel that produces CH30 + NH~ is the dominant channel through a fast intracluster proton transfer reaction. For another channel corresponding to pro- duce CH2OH + NH~-, which follows an intracluster rearrangement reaction, a high energy barrier makes it less favored.

Acknowledgements

This work was supported by the Natural Science Foundation of China.

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