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J. Phys. Chem. 1996, 100, 3430-3434
Density Functional Theory Study of Molecular Structures and Vibrational Spectra of 3,4- and 2,3-Pyridyne Ruifeng Liu,* Dennis R. Tate, Jeffrey A. Clark, Paula R. Moody, Alex S. Van Buren, and Joel A. Krauser Department of Chemistry, East Tennessee State UniVersity, Johnson City, Tennessee 37614-0695 ReceiVed: June 6, 1995; In Final Form: September 11, 1995X
Density functional theory and ab initio MP2 6-31G* calculations were carried out to investigate the structures and vibrational spectra of 3,4- and 2,3-pyridyne. It is found that the structure of 3,4-pyridyne is consistent with a formal CC triple bond moiety, but the structure of 2,3-pyridyne is more properly described as having a C‚‚‚C‚‚‚N unit. On the basis of the calculated results, detailed assignments of the observed IR bands of 3,4-pyridyne are proposed. The calculations predict the most prominent IR feature of 2,3-pyridyne is a very strong band around 1826 cm-1, suggesting the search for direct experimental evidence of 2,3-pyridyne should pay attention to this spectral region.
Introduction Pyridynes (didehydropyridines) have been proposed as intermediates in many organic reactions such as cycloaddition, cine-substitution, and tele-substitution.1 They are the most studied of all known didehydroheteroarenes.1-7 Yet there was no direct evidence for the existence of pyridyne as an intermediate until 1988 when Nam and Leroi published the first matrix infrared study of the photolysis products of 3,4-pyridinedicarboxylic anhydride.8 Before this paper was published, only indirect evidence, based on trapping experiments to verify the presence of heteroaryne, was obtained. Reliability of the indirect evidence is limited, because other reaction mechanisms may also account for the observed products.1 The IR study of Nam and Leroi firmly established 3,4-pyridyne as a photolysis product of 3,4-pyridinedicarboxylic anhydride. To understand the stability of didehydropyridines and the observed IR spectral features, Leroi et al. carried out ab initio 3-21G restricted Hartree-Fock (RHF) and generalized valence bond (GVB) calculations on the lowest singlet and triplet states of six didehydropyridines.9 Although valuable insights into the structures, vibrational spectrum, and stability of these molecules were obtained from these calculations, both the 3-21G basis set and the RHF and GVB theories are not completely satisfactory for studying these molecules. The significant strain in these molecules requires a basis set with polarization functions to give an adequate description of the structural features. The strain is also expected to result in a very small energy gap between the LUMO and HOMO, making effects of electron correlation important factors in describing molecular properties. The GVB theory uses a two-configuration wave function and recovers electron correlation partially. Such a wave function describes the biradical nature of the formal triple bond adequately. It is expected, however, that the whole π-electron system of pyridynes is involved in resonance. For these molecules, an adequate description of electron correlation effects in a multiconfigurational approach requires a wave function consisting of all configurations resulting from distributing 10 electrons (6 out-of-plane and 2 in-plane π electrons plus the lone pair electrons of nitrogen) in 9 active orbitals (6 out-ofplane and 2 in-plane π orbitals plus the lone pair orbital of nitrogen). Calculation with such a wave function is quite expensive and often plagued with convergence problems. X
Abstract published in AdVance ACS Abstracts, February 1, 1996.
0022-3654/96/20100-3430$12.00/0
Recently, the density functional theory (DFT)10 has been accepted by the traditional ab initio quantum chemistry community as a cost-effective approach to molecular properties.11-13 The computational cost of a DFT calculation is similar to that of a Hartree-Fock calculation, yet it recovers electron correlation effectively through the empirical functionals and gives surprisingly good descriptions for many conventional and illbehaved molecules.11-17 Many DFT studies indicate that the combination of Becke’s gradient corrected exchange functional20 and Lee-Yang-Parr’s gradient corrected correlation functional (BLYP)21,22 predicts geometrical parameters of most organic compounds in good agreement with experimental results, with perhaps slight overestimation of bond distances.11-16 Better agreement between experimental structural parameters and calculated results is achieved by hybrid Hartree-Fock/DFT methods.18 One of the most popular hybrid HF/DFT methods is Becke’s three-parameter method in conjunction with LeeYang-Parr’s correlation functional (B3LYP).23 Probably due to error cancellation, the BLYP harmonic frequencies appear in better agreement with observed fundamental vibrational frequencies.16a The B3LYP harmonic frequencies are perhaps in better agreement with true harmonic frequencies, which are not observable in any experiment. In the recent quantum chemistry class at East Tennessee State University, we studied the structures and vibrational spectra of 3,4- and 2,3-pyridyne by DFT methods as a course project. The results are presented in this paper. Calculations All our calculations were carried out using the Gaussian program package.19 The structural parameters of 3,4- and 2,3pyridynes were fully optimized by density functional theory using Becke’s exchange20 and Lee-Yang-Parr’s correlation functional21 as transformed by Miehlich et al.22 (BLYP) and by Becke’s three-parameter hybrid DFT/HF method23 using Lee-Yang-Parr’s correlation functional (B3LYP). Vibrational analyses were carried out on the optimized structures. The 6-31G* basis set24 was used in all the calculations. For comparison, we also carried out similar calculations using the second-order Moller-Plesset perturbation theory (MP2) with the frozen core approximation. It should be noted that MP2 is not a more reliable method than DFT, yet it is the only correlated ab initio method we can apply to these molecules with our © 1996 American Chemical Society
Structures and Spectra of 3,4- and 2,3-Pyridyne
Figure 1. Comparison of the calculated and experimental structural parameters of pyridine (A) and comparison of the calculated structural parameters of o-benzyne (B). The 6-31G* basis set is used in the DFT calculations. The CCSD(T) and MP2 results were taken from ref 26, which were obtained by using the 6-31G(d,p) basis set. The MW structure of pyridine is taken from ref 25a, and the ED + MW structure is taken from ref 25b.
Figure 2. Comparison of the calculated structural parameters of 3,4pyridyne (A) and 2,3-pyridyne (B). The bond lengths are given in angstroms and angles in degrees. The 6-31G* basis set is used in these calculations.
computing facility. As neither experimental nor higher level theoretical structures of the target molecules are available, to test reliability of the DFT results, we also made DFT geometry optimization on pyridine and o-benzyne. These results are compared with available experimental25 and higher level (CCSD(T)) theoretical results.26 Results and Discussion Structures. The DFT structural parameters of pyridine are compared with experimental results25 in Figure 1A, and the DFT structural parameters of o-benzyne are compared with the CCSD(T) and MP2 results26 in Figure 1B. The calculated structural parameters of 3,4-pyridyne are compared with the RHF/3-21G and GVB/3-21G results9 in Figure 2A. The calculated structural parameters of 2,3-pyridyne are compared with the RHF/3-21G and GVB/3-21G results9 in Figure 2B. In these figures, the bond lengths are given in angstroms, and angles are in degrees. Figure 1A indicates that the experimental structural parameters of pyridine are reproduced satisfactorily by both B3LYP and BLYP calculations. The maximum difference between the B3LYP results and the latest combined electron diffraction and microwave study is 0.005 Å in bond length and 1° in bond angles. As expected, the major difference between the B3LYP and BLYP results of pyridine is that all the BLYP bond lengths are slightly (
