MP2 and DFT study of IR spectra of TCNE‐methylsubstituted benzene complexes: Is charge transfer important?

Abstract

AbstractThe second‐order Møller–Plesset (MP2) and density functional theory (DFT; B3LYP, B3LYP‐D) approaches show that the CN (and also CC) stretching IR vibrations are relatively small but reproducibly shifted to lower frequencies in a series of methylbenzenes‐tetracyanoethylene (NMB‐TCNE, N = mono‐, di‐, … , hexa‐) complexes with increasing number of methyl groups (N). These CN stretching IR frequency shifts, ranging from 8 to 12 cm−1, are linearly dependent on number N but still 2‐4 times greater than recently published experimental data [Stires et al., Chem Commun, 2005, 28, 3532; Pawlukojc et al., Chem Phys, 2006, 327, 311]. An explanation of the shifts toward lower frequencies is in literature [Stires et al., Chem Commun, 2005, 28, 3532; Pawlukojc et al., Chem Phys, 2006, 327, 311] usually attributed to the magnitude of transferred electron charge ΔQ from a donor molecule to TCNE (mainly to its π LUMO [lowest unoccupied molecular orbital]). The LUMO has antibonding character on CN and CC bonds, and thus the respective CN and CC bond orders are, due to partial occupation in the complex, decreased. However, the difference by a factor of 2‐4 between our MP2 (and also B3LYP) calculations and experiment is probably also due to the effect of environment, i.e., intermolecular forces between complex and solvent molecules or crystal lattice. The calculated interplanar distance R, dipole moment μ, and transferred charge ΔQ are approximately linearly dependent on number of methyl groups N. However, between N = 2 and 3, the abrupt changes in the dependencies were observed. This is caused by the change of mutual (perpendicular) orientations between respective methyl benzenes and TCNE in the complexes as a consequence of preferred interaction between one of two degenerated or near degenerated π highest occupied molecular orbitals (being orthogonal) on NMB with π LUMO located mainly on TCNE. This fact is important for many properties (especially for electron excited states) of these complexes. A strong dependence of polarizability, namely of its αzz part, of the benzene‐TCNE (Be‐TCNE) complex on soft vibrations was found. The vibrational contribution to αzz value is almost as large as the electronic αzz contribution. Finally, MP2 and B3LYP studies indicate that inclusion of anharmonicity has small, if any, effect on calculated vibrational frequencies of Be‐TCNE. This theoretical finding is quite important especially because it holds also for soft vibrational modes, which are always present at weak intermolecular complexes. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010