Theoretical Studies Of The Intermolecular Interaction And Its Effects On The Nonlinear Optical Properties

(1) The methyl radical (CH₃) complexes with hydrogen fluoride (HF) and ethyne (HCCH) are reported to show the existence of a single-electron hydrogen bond. Their geometrical structures are optimized at the MP2/aug-cc-pVTZ levels and C3v stationary structures are obtained for the two complexes. The single-electron hydrogen bond energies of H₃C···HF and H₃C···HCCH are calculated at six levels of theory [SCF, MP2, MP3, MP4, CCSD, and CCSD(T)] and their harmonic vibrational frequencies are calculated at the MP2/aug-cc-pVTZ level.(2) A new intermolecular covalent interaction is observed theoretically in the tetrathiafulvalence cation radical dimer TTF·⁺–TTF·⁺. This new covalent interaction is a 20-center 2-electronσ-bond which comes from the stacking and bonding of two singly occupiedÏ€molecular orbitals of the two different TTF·⁺ subunits in the dimer (TTF·⁺)₂. The new intermolecularσ-bond between twoÏ€molecular orbitals is denoted asσ²(Ï€/Ï€)σ-bond and counteracts partly the cation-cation electrostatic repulsive interaction between two TTF·⁺ cations. The bond energy of this intermolecularσ-bond is estimated to be 32.2 kcal·mol^-1 which is larger than the negative value of the noncovalentÏ€/Ï€stacking interaction (-4.95 kcal·mol-1 for the benzene dimer).(3) The effects of the intermolecular interaction on dipole moment, the polarizability, the first and second hyperpolarizabilities of Ar–HF have been calculated in finite-field approach. The intermolecular interaction of Ar–HF slightly increases the dipole moment and has rather small effects (<1%) on the polarizability and the second hyperpolarizability. The effect of the intermolecular interaction on the first hyperpolarizability of Ar–HF is rather large and significantly decreases the first hyperpolarizability for this complex.(4) The effects of intermolecular interaction on the dipole moment, the polarizability, and the first and second hyperpolarizabilities for four hydrogen bond dimers between twoÏ€-systems (HCN···HCN, HNC···HCN, HCN···HNC, and HNC···HNC) have been calculated in the finite-field approach. The properties were calculated at the MP4/d-aug-cc-pVTZ level. Intermolecular interaction significantly increases the first hyperpolarizabilities of four dimers between twoÏ€-systems (177% for HCN···HCN, 47.9% for HNC···HCN, 65.3% for HCN···HNC, and 37.9% for HNC···HNC). Intermolecular interaction effects of the second hyperpolarizabilities are 3.0% for HCN···HCN, -3.1% for HNC···HCN, 5.4% for HCN···HNC, and -2.1% for HNC···HNC.(5) Novel cluster anions Li₂F^- and Li6F^- with alkalide character have been studied. In contrast to a typical neutral alkalide, Li₂F^- contains a F^- anion instead of the neutral ligand and forms an alkalide anion F^-Li⁺Li^-. In addition to a F^- anion ligand, Li6F^- contains a Li₃⁺ superalkali cation instead of the alkali metal cation and a Li₃^- superalkali anion instead of the alkali metal anion, and this alkalide anion can be denoted by F^-Li₃⁺Li₃^- that is supported by NBO charge results. The results indicate that the F^- anion can polarize not only the Li atom but also the Li₃ superalkali to form alkalide anions with excess electrons. For Li₂F^-, two linear structures (¹Î£⁺ and ³Î£⁺ states) are obtained. Due to the excess electrons on the alkali metal and superalkali anions (Li- and Li₃-), the alkalide anions Li₂F^- and Li6F^- have large first hyperpolarizabilities (β₀ = 1.116×10⁴～1.764×10⁵ a.u.). For the spin multiplicity effect on electric properties, in these two alkalide anions, the values of the static electric properties, especially the first hyperpolarizabilities, of the high spin states are larger than the corresponding values of the low spin states. For the substitution effect of superalkali atoms, in the two singlet states, as the Li₃ superalkalis substitute the Li atoms, the value of the mean of polarizability increases, while the values of dipole moment and the first hyperpolarizability decrease.This paper richens the new knowledge of the intermolecular interaction, provides a new idea to increase the hyperpolarizabilities of NLO materials, and provides the theoretical basis to design the new NLO materials.