J . Phys. Chem. 1993,97, 3157-3159
3157
Ab Initio Study of the Structure of HCOO-in a Water Cluster Masao Masamura’ Department of Preventive Dentistry, Okayama University Dental School, Shikada-cho 2-5-1, Okayama 700, Japan Received: March 30. 1992
The purpose of this article is to predict the structure of the HCOO- anion in aqueous solution. For this purpose, the following effects of water molecules on the structure of HCOO- in aqueous solution are considered: (A) the effect of the first solvent shell; (B)the effect of water molecules around the 0’of HCOo-(H20’)6; (C) the effect of water molecules around the H’ of H’COo-(H20)6; (D) the effect of water molecules around H’ of HCOO-(HOH’)6. In order to estimate these effects, geometry optimization with 3-21+G and 6-31++G* basis sets is carried out for HCOO-(H20),, (n = 0, 1, 2, 3, 4, 5 , a), HCOO-(HzO)6(HOH), (m = 1, 2), HC0O-(H20)6(OH2), (m = 1, 2), H20 ...HCOO-(H20)6. The effect of the first solvent shell is significant. The other effects are small. Thus, the structure of HCOO- in aqueous solution is nearly the same as the structure of HCOO- in HCOO-(H20)6. The C-H bond length of HCOO- in aqueous solution is smaller than the C-H bond length of HCOO- in the gas phase by 0.017 A.
I. Introduction The structure of RCOO- in aqueous solution is interesting because many chemically and biologically important species such as amino acids and peptides have carboxylate groups in aqueous solution. However, at the present stage of experimentation, it is impossible to determine the structure of RCOO- in aqueous solution. The purpose of this article is to predict the structure of the HCOO- anion in water as the initial step in determining the structure of RCOO- in aqueous solution.
structural changes by 6-31++G*; (3) the 3-21+G optimized structure of HCOO- was compared with the optimized structure of HCOO- by MP3/6-31++G* without frozen core;’ (4) stabilization energy (-AE,+,,,,) was compared with experimental -AHn-l,n.
For the analysis of atomic electron populations, the natural population analysisSwas used. We used the Gaussian 82,3Gaussian 86: and NB07programs and the M-680 and S-820computers at the Institute for Molecular Science. 111. Results and Discussion
11. Method
In order to predict the structure of HCOO- in aqueous solution, we must consider the following effects of water molecules on the structure of HCOO- in aqueous solution: (A) the effect of water molecules in the first solventshell (we assume six water molecules in the first solvent shell, although a Monte Carlo calculation predicted that seven water molecules exist in the first solvent shell’); (B) the effect of water molecules around the 0’ of HC00-(H20’)6; (C) the effect of water molecules around the H’ of H’C00-(H20)6; (D) the effect of water molecules around the H’ of HCOO-(HOH’)b. In order to estimate these effects by the ab initio closed-shell SCFmethod, wecarriedout geometryoptimizationwith 3-21+G2 and 6-31++G*’ basis sets for HC00-(H20), (n = 0,1,2,3,4, 5,6), HCOO-(H20)6(HOH)m ( m = 1,2), HCOO-(H20)6(OH2)m ( m = 1,2), H20...HC00-(H20)6, (HzO), (n = 1,2), and OHin Figure 1. The vibration analysis for HCOO-(H20), (n = 1,2,3,4) with 3-21+G was performed at the optimized structures to confirm all real vibrational frequencies. For the evaluation of reliability of 3-21+G, the following calculations were performed: (1) because the proton affinity difference between HCOO- and OH- is important: the proton affinity difference evaluated by 3-21+G was compared with the proton affinity difference determined by experimental enthalpy; (2) the structural changes of HCOO-(H20), with n increment (n = 0 4 ) evaluated using 3-21+G were compared with the Present address: Akoda 307-5, Okayama 703, Japan.
0022-365419312097- 3 157$04.00/0
Figure 1 shows the optimized structures. 1. The Reliability of 3-21+C. The proton affinity difference evaluated by 3-21+G (AE = 4 7 . 0 kcal/mol) agrees with the proton affinity difference determined by using an experimental enthalpy (AH = 4 5 . 6 kcal/mol).* The changes in structural parameters with n increment in 3-21+G agree with the changes instructural parameter sin 6-31++G*. TheoptimizedC-H bond length of HCOO- with 3-21+G is shorter than the optimized C-H bond length of HCOO- with MP3/6-31++G* by 4 , 0 2 6 A. The optimized C-0 bond length of HCOO- with 3-21+G is longer than the optimized C-O bond length of HCOO- with MP3/6-31++GS byO.O1O A. With thecorrections for the C-H (+0.026 A) and C-O (-0.010 A) bond lengths optimized with 3-21+G, we can obtain acceptable bond lengths for C-H and C-0 in this paper. Table I shows -AE,-I,nin 3-21G is slightly overestimated. 2. Effect of Water Molecules in the First Solvent SheU. Tables I1 and 111show the changes in structural parameters and charges on atoms in HCOO-(H20), with each n increment. When n becomes larger, (1) the C-H bond length becomes smaller, (2) the charge on the H’ of H’COO-(H20), becomes larger, (3) the charge on the C is larger, (4) the C-O bond length and HCO bond angle are nearly unchanged, and (5) the charge on the 0 of HCOO- becomes slightly more negative. We showed the cause for the elongation of the H-C bond in HCOO- in the gas phase.9 This elongation results from the contribution of H-...C02 as resonance structure to the HCOO-.9 When more water molecules attach HCOO-, more minus charge withdraws from HCOO- to water molecules. Consequently, it Q 1993 American Chemical Society
Masamura
3158 The Journal of Physical Chemistry, Vol. 97, No. 13, 1993
TABLE I: Stabilization Ener (-AEel,,,) for HCW(H~O). cluster (in &mi, -AE.-1,. -AH"I .n n
3-21+G
6-3 I ++G*
1
15.5
2
21.0(18.5)c [20.9Id 19.2 (16.6)
