Norifumi Yamamoto

IR-dip and IR-UV hole-burning spectra of jet-cooled 4-aminobenzonitrile-(H2O)1. Observation of π-type and σ-type hydrogen-bonded conformers in the CN site

Abstract The IR-dip spectra and IR–UV hole-burning spectra of jet-cooled 4-aminobenzonitrile–water 1:1 complex have been measured to investigate the effects of the introduction of two substituents into the aromatic ring on the hydrogen-bonding interaction and stable structures of the complex. We have obtained a clear evidence for the observation of three structural isomers by comparing the experimental IR spectra with the theoretical ones. The water molecule is bonded to the NH 2 site in isomer I, where the amino group can act as a proton donor and the amino hydrogen is bonded to the oxygen atom of water. Water is bonded to the CN site in isomers II and III. The structure of isomer II is very similar to benzonitrile–(H 2 O) 1 , where the water hydrogen is bonded to the cyano nitrogen and the oxygen atom of water is bonded to the ortho hydrogen atom. The water hydrogen is linearly hydrogen-bonded to the cyano nitrogen in isomer III. The intermolecular hydrogen bond in isomer II is σ-type, whereas that in isomer III is π-type. The proton-donor conformer in the NH 2 site and the σ-type linear conformer in the CN site have not been observed in the aniline–(H 2 O) 1 and benzonitrile–(H 2 O) 1 complexes, respectively. The observation of three stable structures has been successfully explained by atomic charges on the constituent atoms obtained by natural population analysis.

Electronic and vibrational spectra of aniline-benzene hetero-dimer and aniline homo-dimer ions

Abstract Structures of (aniline–benzene) + and (aniline) 2 + are re-investigated by electronic spectroscopy in the near-infrared region and vibrational spectroscopy in the NH stretching region. The spectra of (aniline–benzene) + indicate a structure including a hydrogen bond between an NH bond of the ionic aniline and the π-electrons of the neutral benzene. Two isomers are suggested for (aniline) 2 + in which an NH bond of the ionic aniline forms different types of hydrogen bond with the neutral aniline: one with the π-electrons of the aromatic ring and the other with the lone pair of the nitrogen atom.

Electronic and infrared spectra of jet-cooled 4-aminobenzonitrile-H2O. Change of NH2 from proton acceptor to proton donor by CN substitution

Abstract The electronic and infrared spectra of jet-cooled 4-aminobenzonitrile–(H2O)1 (4ABN–(H2O)1) hydrogen-bonded complex have been measured by the resonance-enhanced multiphoton ionization (REMPI) and infrared-dip (IR-dip) spectroscopy. Both the amino and cyano groups form intermolecular hydrogen bond with water, providing two stable isomers. It has been found that the substitution of the CN group at the para-position of aniline changes the electronic nature of the amino group in the S0 state from a proton acceptor to a proton donor.

External Electric Field Effects on Excited-State Intramolecular Proton Transfer in 4'-N,N-Dimethylamino-3-hydroxyflavone in Poly(methyl methacrylate) Films.

The external electric field effects on the steady-state electronic spectra and excited-state dynamics were investigated for 4-N,N-(dimethylamino)-3-hydroxyflavone (DMHF) in a poly(methyl methacrylate) (PMMA) film. In the steady-state spectrum, dual emission was observed from the excited states of the normal (N*) and tautomer (T*) forms. Application of an external electric field of 1.0 MV·cm(-1) enhanced the N* emission and reduced the T* emission, indicating that the external electric field suppressed the excited-state intramolecular proton transfer (ESIPT). The fluorescence decay profiles were measured for the N* and T* forms. The change in the emission intensity ratio N*/T* induced by the external electric field is dominated by ESIPT from the Franck-Condon excited state of the N* form and vibrational cooling in potential wells of the N* and T* forms occurring within tens of picoseconds. Three manifolds of fluorescent states were identified for both the N* and T* forms. The excited-state dynamics of DMHF in PMMA films has been found to be very different from that in solution due to intermolecular interactions in a rigid environment.

Density functional studies on aniline dimer cations

Abstract The structures of aniline dimer cation are investigated by the density functional theory calculations at the B3LYP/cc-pVDZ level. We obtained two stable conformational isomers which have `NH⋯N or `NH⋯π hydrogen bond. The NH⋯N isomer is more stable by 2.50 kcal mol −1 . The calculated infrared spectra suggest that the NH stretching vibrations due to the two isomers are overlapped in the experimental spectrum [Chem. Phys. Lett. 323 (2000) 43].

Hole-burning spectroscopy and ab initio calculations for the aniline dimer

Abstract The mass-resolved spectrum indicates that only one conformation contributes to sharp peaks observed in the S 1 ←S 0 resonance two-photon ionization (R2PI) spectrum. However, geometry optimizations at the MP2/cc-pVDZ level suggest that two conformational isomers are stable: a head-to-head conformation with a single NH⋯N hydrogen bond and a head-to-tail conformation with double NH 2 ⋯π hydrogen bonds. The calculations show that the head-to-tail conformation is more stable by 1.18 kcal mol −1 .

A theoretical study of strong anharmonic coupling between OH stretching and bending modes in phenol-water cationic complex

Abstract The OH stretching vibrations of phenol (Ph), phenol cation ([Ph] + ), phenol–water complex (Ph–H 2 O) and phenol–water cationic complex ([Ph–H 2 O] + ) are investigated theoretically in terms of ab initio potential energy surfaces, vibrational wavefunctions, and infrared absorption intensities. It was found that the anharmonic coupling between the hydrogen-bonded OH stretching and bending vibrations in [Ph–H 2 O] + is much stronger than those in Ph, [Ph] + and Ph–H 2 O, resulting in vibrational wavefunction mixing and infrared intensity borrowing.

Hole-burning spectra of tropolone–(CO2)n (n=1,2) van der Waals complexes and density functional study

Abstract The hole-burning, fluorescence excitation, and dispersed fluorescence spectra of jet-cooled tropolone (TRN)–(CO 2 ) n ( n =1,2) complexes are measured to investigate the structures of the complexes and the effects of intermolecular interaction on proton tunneling in TRN. The electronic transitions of TRN–(CO 2 ) n ( n =1,2) are well separated in the hole-burning spectrum. Only the transitions of one species have been identified for the 1:1 and 1:2 complexes. Structures of TRN–(CO 2 ) n ( n =1,2) are optimized by the density functional theory calculations at the B3LYP/cc-pVDZ level. Three local minima have been obtained for both the 1:1 and 1:2 complexes. In the 1:1 complex CO 2 is bonded in the molecular plane close to a solvation site, Cue605O (Isomer I), Cue605O⋯H–O (Isomer II), or Cue605O (Isomer III). These complexes are stabilized mainly by the dipole–quadrupole interaction, and the binding energies for these complexes are estimated to be much smaller than those for the hydrogen-bonded complexes. Isomers I and II are plausible candidates for the observed species. The calculations imply that asymmetry of the double-minimum potential well along the tunneling coordinates of TRN–(CO 2 ) 1 is large enough to quench proton tunneling. This prediction is consistent with the nonobservation of the tunneling splittings even for the excitation of the tunneling promoting mode ν 13 (a 1 ) or ν 14 (a 1 ) of TRN.

Classical trajectory calculations of intramolecular vibrational energy redistribution. I: Methanol-water complex

Intramolecular vibrational energy redistributions of the O-H stretching (nuOH) vibration for the methanol monomer and its water complex, the methanol-water dimer, are investigated by using ab initio full-dimensional classical trajectory calculations. For the methanol monomer, in the high-energy regime of the 5nuOH overtone, the time dependence of the normal-mode energies indicates that energy flowed from the initial excited O-H stretching mode to the C-H stretching mode. This result confirms the experimental observation of energy redistribution between the O-H and C-H stretching vibrations [L. Lubich et al., Faraday Discuss. 102, 167 (1995)]. Furthermore, a lot of dynamical information in the time domain is contained in the power spectra, whose density is given by the Fourier transformation of the total momentum obtained from trajectory calculations. For the methanol-water hydrogen-bonded complex, at the high-energy level of the 5nuOH overtone, the calculated power spectrum shows considerable splitting and broadening, indicating significant energy redistribution through strong coupling between the O-H stretching vibration and other vibrations. It is thus clear that the A-H...B hydrogen-bond formation facilitates energy redistribution subsequent to the vibrational excitation of the hydrogen-bonded A-H stretching mode.

Excited-state intramolecular proton transfer and conformational relaxation in 4′-N,N-dimethylamino-3-hydroxyflavone doped in acetonitrile crystals

The effect of intermolecular interactions on excited-state intramolecular proton transfer (ESIPT) in 4-N,N-dimethylamino-3-hydroxyflavone (DMHF) doped in acetonitrile crystals was investigated by measuring its temperature dependence of steady-state fluorescence excitation and fluorescence spectra and picosecond time-resolved spectra. The relative intensity of emission from the excited state of the normal form (N*) to that from the excited state of the tautomer form (T*) and spectral features changed markedly with temperature. Unusual changes in the spectral shift and spectral features were observed in the fluorescence spectra measured between 200 and 218 K, indicating that a solid-solid phase transition of DMHF-doped acetonitrile crystals occurred. Time-resolved fluorescence spectra suggested conformational relaxation of the N* state competed with ESIPT after photoexcitation and the ESIPT rate increased with temperature in the low-temperature phase of acetonitrile. However, the intermolecular interaction of N* with acetonitrile in the high-temperature phase markedly stabilized the potential minimum of the fluorescent N* state and slowed the ESIPT. This stabilization can be explained by reorganization of acetonitrile originating from the strong electric dipole-dipole interaction between DMHF and acetonitrile molecules.