Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
A theoretical study of hydrogen complexes of the X
H-π type between propyne and HF, HCL or HCN
Abstract
The present manuscript reports a systematic investigation of the basis set dependence of some properties of hydrogen-bonded (π type) complexes formed by propyne and a HX molecule, where X = F, Cl and CN. The calculations have been performed at Hartree–Fock, MP2 and B3LYP levels. Geometries, H-bond energies and vibrational have been considered. The more pronounced effects on the structural parameters of the isolated molecules, as a result of complexation, are verified on RC
C and HX bond lengths. As compared to double-ζ (6-31G**), triple-ζ (6-311G**) basis set leads to an increase of RCC bond distance, at all three computational levels. In the case where diffuse functions are added to both hydrogen and ‘heavy’ atoms, the effect is more pronounced. The propyne–HX structural parameters are quite similar to the corresponding parameters of acetylene–HX complexes, at all levels. The largest difference is obtained for hydrogen bond distance, RH, with a smaller value for propyne–HX complex, indicating a stronger bond. Concerning the electronic properties, the results yield the following ordering for H-bond energies, ΔE: propyne⋯HF > propyne⋯HCl > propyne⋯HCN. It is also important to point out that the inclusion of BSSE and zero-point energies (ZPE) corrections cause significant changes on ΔE. The smaller effect of ZPE is obtained for propyne⋯HCN at HF/6-311++G** level, while the greatest difference is obtained at MP2/6-31G** level for propyne⋯HF system. Concerning the IR vibrational it was obtained that larger shift can be associated with stronger hydrogen bonds. The more pronounced effect on the normal modes of the isolated molecule after the complexation is obtained for HX stretching frequency, which is shifted downward.
Introduction
T-shaped hydrogen-bonded complexes of the type XH-π, formed by XH hydrogen halide and π-electron density of carbon–carbon double or triple bond, with XCl, F or CN, constitute very important systems since they are involved in the first step of electrophylic addition reactions to an unsaturated hydrocarbon [1]. It is well known that the complexation causes various changes in the molecular spectrum, which make such techniques suitable for characterization. Among them are microwave and infrared molecular beam experimental techniques with Fourier transform strength [2], [3], [4], [5]. For example, for H-bonded complexes involving acetylene and HX it is obtained an enhancement in the stretching intensities associated with chemical bonds directly involved in the H-bond formation and shifts in their vibrational frequencies [6]. Theoretical investigations using ab initio calculations have been successfully applied in order to understand the nature of the hydrogen bonding as well as changes in the structural, electronic and vibrational properties that take place in the HX and acetylene moieties after molecular complexation [7], [8], [9]. On the other hand, theoretical calculations have also been particularly useful to identify the new low-frequency vibrational modes that have, in general, very weak intensities.
This work reports ab initio results for hydrogen complexes involving the propyne molecule as proton acceptor and monoprotic linear acids as proton donors. One of the main tasks is to get a deeper insight into the factors controlling the formation of hydrogen-bonds, through a comparative study with the corresponding acetylene–HX complexes. Such comparison can help to get a better understanding of how the increase of the backbone alters the structural and electronic properties of unsaturated hydrogen-bonded hydrocarbons.
The choice of appropriate quantum chemistry methods and basis set in order to get a correct description of weakly bonded systems is still a difficult task. Herein, a systematic investigation using ab initio molecular orbital calculations at restricted Hartree–Fock (RHF) [10], MP2 [11], and B3LYP [12] level with several Pople basis set [13], [14] have been performed. The goal is to understand the effect of electronic correlations methods and basis set size on geometrical, electronic and vibrational calculated parameters. Other important aspect is the error due the LCAO approximation; called basis set superposition error (BSSE). There are several methods to estimate the BSSE. The well-established counterpoise method developed by Boys and Bemardi [15] is chosen for the present work. The calculations were carried out using the Gaussian 98W program [16].
Section snippets
Structural parameters
The select geometry parameters for the propyne⋯HCl, propyne⋯HCl and propyne⋯HCN optimized at Hartree–Fock, MP2 and B3LYP levels, with several basis set, are given in Table 1. An enlargement of the RCC bond distance is observed with the inclusion of diffuse functions in 6-31G** and 6-311G** basis set. As expected, the effect is greater for the smaller (6-31G**) basis set, at all computational levels studied. The RHX bond distance changes by few thousandths of angstroms with basis set enlargement
Conclusions
The basis set dependence of some electronic and structural properties of XH-π type hydrogen complexes formed between propyne and HF, HCl and HCN have been studied at Hartree–Fock, MP2 and B3LYP levels. The stabilization energies, ΔE, as well as the corrected stabilization energies, ΔECORR, which includes the BSSE and ZPE corrections, computed at Hartree–Fock and B3LYP levels, show that the BSSE results decrease with the basis set size. However, the Hartree–Fock values of ΔE, BSSE and ZPE are
Acknowledgements
The authors gratefully acknowledge partial financial support from the Brazilian funding agencies CNPq, CAPES and FINEP.
References (17)
- et al.
Spectrochim. Acta. A
(1995) - et al.
J. Mol. Struct. Theochem.
(1996) - et al.
Spectrochim. Acta. A
(2001) - et al.
J. Am. Chem. Soc.
(1992) - et al.
J. Chem. Soc., Faraday. Trans.
(1988) - et al.
J. Chem. Phys.
(1987) - et al.
Chem. Rev.
(1986) - et al.
J. Chem. Phys.
(1982)