Ab initio study of the HCOOHAr van der Waals complex

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21 February 1997

CHEMICAL PHYSICS LETTERS ELSEVIER

Chemical Physics Letters 266 (1997) 1-6

Ab initio study of the HCOOH-Ar van der Waals complex Jan Lundell Laboratory of Physical Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, (A.I.Virtasen Aukio 1), FIN-O0014 Helsinki, Finland

Received 25 November 1996

Abstract

The structure, energetics and vibrational properties of the van der Waals complex between HCOOH and Ar have been studied by ab initio molecular orbital theory. Three local minima, two in the plane of HCOOH and one non-planar, could be found with interaction energies ranging between - 1.3 and - 1.9 kJ mol-1. The vibrational properties of HCOOH were found to be unperturbed except in the case of the interaction from the OH-tail. The shift in the OH absorption should enable the identification of this complex structure easily from the IR spectrum.

1. I n t r o d u c t i o n The study of weakly bound clusters of atoms and molecules has grown rapidly in recent years [1-3]. Experimental and theoretical interest in weak interactions has been fuelled by several questions of fundamental importance in chemical physics. How molecules initially select one of several reaction channels, how molecules bind together in complex biochemical systems, how molecules are oriented on surfaces and in small aggregates, are all questions of small energetic differences related to the long-range influence of the electric field characteristic of the molecule. The van der Waais complexes between molecules and rare gas atoms are of fundamental importance to the understanding of weak chemical interactions. Rare gas atoms can act as simple, spherical probes of the shape of the molecule's charge distribution. Moreover, already complexes with one

rare gas atom can act as a model for solvation effects on the chemical reactivity [4,5]. Formic acid is the first and simplest member of the organic acids and it represents an ideal model compound for understanding more complicated molecules. Apart from being an important industrial product, H C O O H is known to play a role in human metabolism, and in atmospheric as well as interstellar chemistry. The electric field produced by the carbonyl and hydroxyl groups should markedly influence its long-range interactions. In this Letter, the first ab initio results for the H C O O H - A r van der Waals complex are presented. One microwave study exists on the H C O O H - A r complex [6]. According to measurements the complex is planar with an argon atom distances of 3.71 and 2.81 A from the carbonyl oxygen and acidic hydrogen, respectively. The binding energy of the complex was estimated to be - 1.73 kJ m o l - I from the centrifugal distortion data

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J. Lundell/Chemical Physics Letters 266 (1997) 1-6

when a Lennard-Jones potential was used in a simple dispersive model.

I

2. Computational methods All calculations were performed within the framework of the ab initio closed shell approximation using the GAUSSIAN 94 [7] package of computer codes. The complex properties were considered via the supermolecular Moller-Plesset perturbation theory to second (MP2) order [8,9]. The applied basis set was split-valence 6-311-type Gaussian functions [10] added with diffuse [11] and polarization [12] functions on all atoms to give the 6-311 + + G(2d,2p) basis set. The harmonic wavenumbers were calculated analytically for both the HCOOH monomer and the complexes. The interaction energy was estimated as the difference in total energy between the complex and the monomers at infinite distance, where the monomer wavefunctions were derived in the dimer centered basis set (DCBS). This approach corresponds to the counterpoise correction (CP) proposed by Boys and Bernardi [13] aimed at minimizing the basis set superposition error, BSSE, in the interaction energy. The interaction energy was also evaluated using a larger 6-311 + + G(3df,3pd) basis set at the MP2 level of theory and at third (MP3) and fourth-order (MP4SDTQ) perturbation theory. In these higher order calculations the optimized MP2/6-311 + + G(2d,2p) structure was used. All calculations were carried out on SGI Power Challenge and CRAY C94 supercomputers at the CSC - Center for Scientific Computing Ltd (Espoo, Finland).

3. Results and discussion From the microwave data it has been concluded that the H C O O H - A r van der Waals complex possesses a planar structure with an almost linear O - H Ar bond. The experimental value for the O - H - A r bending angle is 168 °, and for the H - A r bond the atom distance is 2.81 ~, [6]. Computationally, three stable structures could be found, as shown in Fig. 1. Two of these structures are planar H C O O H - A r com-

/ /

II

i /

0 III Pig. 1. The optimized structures of the HCOOH-Ar complexesat the MP2/6-311 + + G(2d,2p) level of theory.

plexes (I and II), and the third one shows the argon atom above the formic acid plane and perpendicular to the C--O bond. The structural parameters for all three structures are given in Table 1. Structure I shows a hydrogen-bonded complex typical of HCOOH. The acidic O - H tail has been found to be an active site in complexes like H C O O H - C O and H C O O H - N 2 [14,15], where the almost linear hydrogen-bonded structure represents the global minimum on the various potential energy surfaces. This structure also resembles the local minimum found computationally for the H 2 0 - A r complex [ 16], even though the global minimum structure was found to be a bifurcated T-shaped complex. Structure II represents a similar higher energy structure that was found for the H C O O H - C O complex, where the interaction occurs with the carbonyl oxygen lone electron pairs. Structure III resembles the local minimum structure found in the case of the H 2 C O - A r complex by Sadlej and co-workers [17]. Now the interaction is formed with the p-electron cloud in the C - O bond. The C - O bond is a typical electron-deficient location due to the vicinity of the polar carbonyl group. This can also be noted from the calculated partial charges in Table 2. In struc-

J. Lundell / Chemical Physics Letters 266 (1997) 1-6 Table 1 Calculated structural parameters a) of H C O O H - A r at the M P 2 / 6 - 3 1 1 + + G(2d,2p) level of theory HCOOH MW ~ Oj-C C-O 2 H t-O 2 H 2-C Ar-H I Ar-O I Ar-C

HCOOH-Ar Calc.

1.203 1.342 0.972 1.097

MW c

1.2040 1.3484 0.9666 1.0896 2.81

I

I! 1.2042 1.3479 0.9669 1.0897 2.7044

!II 1.2041 1.3482 0.9667 1.0896

1.2039 1.3484 0.9666 1.0896

3.4276 3.541l

O I- C - O 2 HI-O2-C H2-C-O 2 Ar-HI-O 2 Ar-OI-C Ar-C-O I

124.82 106.34 111.97

H I-O2-C-H 2 O I-C-O2-H I Ar-H l-O2-C Ar-Oj-C-O 2 Ar-C-O2-O I

180.0 0.0

125.063 106.754 109.783

125.073 106.809 109.820 176.215

125.053 106.750 109.790

125.057 106.726 109.772

176.961 97.238 180.0 0.0 0.0

180.0 0.0

180.0 0.0 0.0

180.0 0.0

180.0 0.0

0.0 90.661

A/MHz B/MHz C/MHz

12116.6 1663.5 1459.4

12616.0 1523.4 1359.2

41953.2 1061.6 1035.4

10497.0 1792.1 1591.0

a The bond lengths are in /tngstr~m and the angles are in degrees. b From Ref. [18]. c From Ref. [6].

tures I and III the partial charge of Ar is positive whereas in the interaction with the electron-rich C = O group it is negative. The optimized structures of the H C O O H - A r complex show an almost undisturbed HCOOH monomer. The largest deviations from the calculated monomer values are the lengothening of the O j - C bond (from 1.2040 to 1.2042 A), and the shortening

Table 2 The calculated partial charges of H C O O H - A r

Ol C 02 HI H2 Ar

1

II

III

- 0.414 0.379 - 0.335 0.249 0.106 0.014

- 0.447 0.411 - 0.339 0.270 0.110 - 0.006

- 0.394 0.311 - 0.330 0.276 0.128 0.009

of the C - O 2 bond (from 1.3484 to 1.3479 ~,) in structure I. For the same structure the interaction distance is 2.70/~ which is close to the experimental value of 2.81 ,~. For the two other structures the interaction distances are predicted to be much longer being close to 3.5,4,. Also the O 2 - H - A r angle in I is predicted to deviate from linearity by a few degrees. However, in the microwave study the Ar atom was found to lie between the OH-tail and the carbonyl group of HCOOH. Computationally, two separate structures with a localized Ar atom either at the carbonyl group (II) or at the OH-tail (I) was found. (See Fig. 1). Similarly to the H 2 0 - A r and H 2 C O - A r complexes [2,16,17], the anisotropy of the potential energy surface is most likely to be substantial. The PES is flat and to locate a minimum with an automatized optimization routine without correlating for the BSSE is bound to have uncertainties in the intermolecular angular dependence.

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J. Lundell/Chemical Physics Letters 266 (1997) 1 - 6

The energetics of the three H C O O H - A r structures are presented in Table 3. The global minimum is structure I at the MP2/6-311 + + G(2d,2p) level of theory. Structures II ( - 0 . 7 6 5 kJ mol -~) and III ( - 1 . 0 5 1 kJ mol -~) represent local minima on the H C O O H - A r PES. Nevertheless, the structure III interaction energy is close to that of I, which indicates that the argon atom can probe the HCOOH electric field almost without energy barriers. In fact, the almost barrierless PES could be the reason for the observation that in the microwave experiments two almost similar structures were found. In both structures the argon atom lies in the plane of HCOOH between the carbonyl group and the OH-tail being slightly tilted either towards the carbonyl oxygen or towards the acidic hydrogen. Computationally the two structures I and I differ only by 0.5 kJ mol -~. The barrier to this 'resonance' can be considered so small that the thermal energy is enough to overcome it. The BSSE-corrected interaction energy for the global minimum (I) is - 1 . 2 3 2 kJ mol -~, which is almost 0.5 kJ m o l - ~ lower than the value of - 1.728 kJ mol-i obtained from molecular jets [6]. This means that only 60% of the experimental interaction energy is reached at the MP2/6-311 + + G(2d,2p) level of theory. Introducing higher order perturbation theory the interaction energy goes down with a

minimum at - 0 . 6 4 8 kJ mol-1 at the M P 4 D Q / 6 311 + + 6(2d,2p)//MP2/6-311 + + 6(2d,2p) level. Bringing triple excitation contributions to the MP4-calculations (MP4SDTQ) the interaction energy results are close to the MP2-calculated value. A similar trend can be found for all three structures and the MP3 and MP4 levels do not bring any changes to the quantitative picture of the interaction at the MP2 level. In the molecule-Ar interaction the multipole electrostatic energy is absent. The dispersion and induction energies dominating the long-range interaction behave variationally with respect to basis set enlargement and should eliminate one important source of basis set effect. Also, the calculated interaction energies should be upper limits to the true energetics. To saturate the slow convergence of the angular expansion of the dispersion energy, diffuse valence orbitals and polarization functions on a double or triple-zeta basis set should be used [2]. Therefore, the interaction energies for the three H C O O H - A r complexes were estimated at the MP2/6-311 + + G(3df,3pd) level of theory. Indeed, the interaction energies can be observed to grow from the M P 2 / 6 311 + + G(2d,2p) level to become close to the experimental interaction energy of - 1.728 kJ mol - t . For the global minimum I the interaction energy is estimated to be - 1.927 kJ mol-~, the structure III is

Table 3 T h e B S S E - c o r r e c t e d i n t e r a c t i o n e n e r g i e s for H C O O H - A r I

(in kJ m o l - t ) I1

III

6-311 + + G ( 2 d , 2 p )

Eel (Eh)

- 716.5124857

-716.5123133

- 716.5124327

0.0

+ 0.453

+0.139

MP2

- 1.232

- 0.765

-

MP3

- 0.879

- 0.425

MP4D

- 0.848

- 0.462

MP4DQ

- 0.648

-0.340

MP4SDQ

- 0.748

- 0.445

MP4SDTQ

- 1.133

-0.818

Ere I (kJ m o l - l)

1.051

6-311 + + G ( 3 d f , 3 p d )

Eeb (Eh) EreI (kJ mol- l)

- 716.6556603

- 716.6550081

- 716.6554805

0.0

+ 1.712

+ 0.472

MP2

1.927

- 1.299

- 1.783

Exp.

- 1.728 a

F r o m Ref. [6].

J. Lundell / Chemical Physics Letters 266 (1997) 1-6

5

Table 4 Observed and calculated vibrational frequencies (in c m - ~) HCOOH-Ar

HCOOH

VoH vcH Vc=o C - H rock C O - C O H deform. Vc_o C - H wag oop ~co H OCO scissors

Gas a

Ar b

Calc.

I

H

Ill

3570.0 2943.8 1776.2 1387 1223.0 1105.4 1033.4 641.8 625.4

3550.4 2952.9 1767.2 1381.0 1215.8 1103.4 1037.4 635.2 627.9

3783.9 3134.1 1788.7 1426.6 1316.8 1123.3 1065.3 676.6 631.7

3777.4 3132.6 1787.4 1427.3 1319.4 1125.6 1065.9 686.7 632.8 43.7 37.8 14.8

3783.3 3134.6 1788.3 1426.6 1316.9 1123.9 1066.1 676.9 632.3 38.7 31.7 19.7

3783.6 3134.0 1789.2 1426.8 1316.8 1123.1 1065.5 676.1 631.6 43.6 30.8 13.1

a From Refs. [19,20]. b From Ref. [15].

almost equal with the experimental value and the highest-energy species (II) at - 1 . 2 9 9 kJ mol-~. Altogether, the interaction energies of the three structures are estimated to be within 0.5 kJ tool-1 of the experimental value at the M P 2 / 6 - 3 1 1 + + G ( 3 d f , 3 p d ) / / M P 2 / 6 - 3 1 1 + + G(2d,2p) level of theory. The vibrational frequencies for all three structures, as well as the observed and calculated values for the formic acid monomer, are presented in Table 4. It can be noted that the largest perturbation for the HCOOH monomer going from the gas phase to the solid argon matrix is observed for the O - H stretching mode. Similarly, the calculated spectra reveal a substantial shift to lower wavenumbers upon complexation to the OH tail. Also, the vibrational modes associated with the movement of the OH-tail (COH torsion and C O - C O H deformation) show larger shifts compared with the monomer values than the other modes. In the solid argon matrix the rare gas cage would restrict most of the movements of the OH-tail. Even the perturbation of the whole argon cage must be accounted for, the matrix data also show a pronounced effect of a H C O O H - A r 1:1 complex with a structure resembling I.

4. Summary The computational data show that the argon-formic acid complex favours a planar structure with an

almost linear hydrogen bond. Along with the global minimum, two local minima were found: one representing an interaction with the carbonyl oxygen and with the argon atom situated above the plane of HCOOH. The experimental estimation of the strength of the interaction in the H C O O H - A r complex was well reproduced from the MP2/6-311 + + G(3df,3pd) level calculation utilizing the M P 2 / 6 311 + + G(2d,2p) optimized structure compared with the experimental data. The calculated vibrational spectra for the complexes reveal the possibility of 'isolating' the H C O O H - A r 1:1 complex interactions in argon matrix experiments.

Acknowledgements The author gratefully acknowledges the CSC Center for Scientific Computing Ltd. for generously providing computer time and excellent computer facilities.

References [1] N. Halberstadt and K.C. Janda, eds., NATO ASI Ser. B 227. Dynamics of polyatomic van der Waals complexes (Plenum, New York, 1990). [2] G. Chalasinski and M.M. Szczesniak, Chem. Rev. 94 (1994) 1723. [3] Z. Bacic and R.E. Miller, J. Phys. Chem. 100 (1996) 12945.

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J. Lundell/Chemical Physics Letters 266 (1997) 1-6

[4] R.B. Gerber, A.B. McCoy and A. Garcia-Vela, Annu. Rev. Phys. Chem. 45 (1994) 275. [5] R.B. Gerber, A.B. McCoy and A. Garcia-Vela, in: Femtosecond chemistry, J. Manz and L. WSste, eds. (VCH, Weinheim, 1995). [6] I.I. loannou and R.L. Kuczkowski, J. Phys. Chem. 98 (1994) 223 I. [7] M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T. Keith, G.A. Petersson, J.A. Montgomery, K. Raghavachari, M.A. AI-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. DeFrees, J. Baker, J.P. Stewart, M. Head-Gordon, C. Gonzalez and J.A. Pople, GAUSSIAN 94, Revision B.I (Gaussian, Pittsburgh, PA, 1995). [8] C. M~ller and M.S. Plesset, Phys. Rev. 46 (1934) 618. [9] J.S. Binkley and J.A. Pople, Int. J. Quantum Chem. 9 (1975) 229. [10] R. Krishnan, J.S. Binkley, R.S. Seeger and J.A. Pople, J. Chem. Phys. 72 (1980) 650.

[11] T. Clark, J. Chandrasekhar, G.W. Spitznagel and P.v.R. Schleyer, J. Comput. Chem. 4 (1983) 294. [12] M. Frisch, J.A. Pople and J.S. Binkley, J. Chem. Phys. 80 (1984) 3265. [13] S.F. Boys and F. Beruardi, Mol. Phys. 19 (1970) 553. [14] J. Lundell, M. R~isS.nen and Z. Latajka, J. Phys. Chem. 97 (1993) 1152. [15] J. Lundell, M. R~isiinen and Z. Latajka, Chem. Phys. 189 (1994) 245. [16] G. Chalasinski, M.M. Szczesniak and S. Scheiner, J. Chem. Phys. 94 ( 1991) 2807. [I 7] J. Sadlej, M.M. Szczesniak and G. Chalasinski, J. Chem. Phys. 99 (1993) 5211. [ 18] R.W. Davis, A.G. Robiette, M.C.L. Gerry, E. Bjarnov and G. Winnewisser, J. Mol. Spectrosc. 81 (1980) 93. [19] R.C. Milliken and K.S. Pitzer, J. Chem. Phys. 27 (1957) 1305. [20] I.C. Hisatsune and J. Heicklen, Can. J. Spectrosc. 18 (1973) 135.