Haruki Ishikawa

Semiclassical study of the isomerization states of HCP

The vibrational spectrum of HCP (phosphaethyne) is studied and analyzed in terms of a 1:2 resonance effective Hamiltonian. The parameters of the model Hamiltonian are determined by fitting 361 out of the first 370 energy levels obtained from diagonalization of the full Hamiltonian, which is based on a newly calculated potential-energysurface with near spectroscopic accuracy. It is demonstrated that all features characteristic of the approach to the HCP↔CPH isomerization, such as the strong mixing between the bending and CP-stretching motions, the appearance of “isomerization states” (large amplitude bending motion) at intermediate energies, and the diagnostically significant appearance of a zig–zag pattern in the energy spacings between neighboring levels within each polyad, are quantitatively reproduced by the effective Hamiltonian. The semiclassical analysis of the model Hamiltonian for specific combinations of the HC-stretch and polyad quantum numbers explains all of the observed features of the full Hamiltonian in terms of stable and unstable periodic orbits. In particular, the birth of the isomerization states is found to be related to a saddle-node bifurcation of the classical phase space. The connection with the “polyad phase sphere” representation of quantum polyads is also discussed.

Photodissociation and spectroscopic study of cold protonated dipeptides.

A photodissociation spectrometer, containing a spray ionization source and a temperature-variable multipole ion trap, has been constructed to examine the structure and reactivity of gas phase biological molecular ions at various temperatures. Ultraviolet (UV) and infrared (IR) photodissociation spectra of protonated alanyltryptophan (Ala-TrpH+) and tryptophanylglycine (Trp-GlyH+) have been measured. In UV spectra, the S1-S0 band origin of Ala-TrpH+ exhibits a significant red shift with respect to those of protonated tryptophan (TrpH+) and Trp-GlyH+. This red shift is ascribed to the stabilization of the excited state due to the strong interaction between the NH3+ group and indole ring. We also discuss the temperature effect on the structure and reactivity for these peptides. In addition to the UV photodissociation spectra of the dipeptides, IR spectra of the complex of Ala-TrpH+ with methanol are measured. IR photodissociation spectra of solvated ions show that Ala-TrpH+-methanol has the closed structure, which is consistent with the large spectral shift in UV spectrum of bare dipeptide.

Spectroscopic investigation of the generation of “isomerization” states: Eigenvector analysis of the bend-CP stretch polyad

The highly excited vibrational levels of HCP exhibit a regular energy level and intensity pattern characteristic of 2:1 bend-CP stretch polyads. Stimulated by the experimental observation of vibrational levels with rotational constants (B-values) 5%–10% larger than other observed levels, Schinke and co-workers noticed that these large-B levels were characterized by atypical nodal structures indicative of large amplitude motion along the minimum energy HCP↔CPH isomerization path [J. Chem. Phys. 107, 9818 (1997)]. In this paper, we show that the transition from “normal-mode-type” to “isomerization” vibrational states arises naturally out of a traditional spectroscopic (algebraic) effective Hamiltonian polyad model. A global least squares fit, based on this polyad Heff model, shows that all of the observed “isomerization” states belong to polyads and that the eigenvectors of this Heff model have the qualitatively distinct nodal structure first noticed by Schinke and co-workers. The “isomerization” states are...

Observation of the “isomerization states’’ of HCP by stimulated emission pumping spectroscopy: Comparison between theory and experiment

Highly excited vibrational states of HCP X 1∑+ were investigated by stimulated emission pumping spectroscopy. Two distinct families of vibrational states were observed in the 13400–17500 cm−1 energy region. One of them is “normal-mode-type” states, well characterized by polyads involving the bending (ω2) and CP stretching (ω3) vibrations. The other is delocalized “isomerization states” whose existence has been predicted by a recent theoretical study [J. Chem. Phys. 104, 10055 (1996)]. The present spectroscopic observations agree well with the theoretical predictions.

Microsolvation and Protonation Effects on Geometric and Electronic Structures of Tryptophan and Tryptophan-Containing Dipeptides

Photodissociation spectroscopy of solvated clusters of protonated tryptophan (TrpH(+)) and dipeptides containing tryptophan (Val-TrpH(+), Ala-TrpH(+), and Gly-TrpH(+)) has been carried out at low temperature to investigate the protonation and solvation effects on the electronic spectrum. For the protonated dipeptides, the S(1)-S(0) transition exhibits a substantial red shift due to the stronger interaction between the NH(3)(+) group and the indole pi ring. The S(1)-S(0) spectra of TrpH(+)(CH(3)OH)(n) clusters exhibit a drastic change with the number of methanol molecules. This behavior is interpreted in terms of the decrease in the interaction between the pi pi* and the repulsive pi sigma* states. Ala-TrpH(+) and Gly-TrpH(+) exhibit an extensive spectral change with addition of two methanol molecules. This change is ascribed to a conformational change, which is induced by the insertion of solvent molecule in between the NH(3)(+) group and the indole pi ring.

Infrared spectroscopy of jet-cooled tautomeric dimer of 7-azaindole: a model system for the ground-state double proton transfer reaction.

To investigate the ground-state double proton transfer (GSDPT) reaction, we carried out a laser spectroscopic study on the tautomeric dimer of 7-azaindole in a supersonic jet. We have recorded an infrared (IR) spectrum of the tautomeric dimer in the S(0) state. The NH band exhibits a broad and less-structured pattern. The band pattern is discussed on the basis of the hierarchical vibrational interaction mechanism. As a result, much higher density of state (DOS) at the NH stretch level is expected than that of the normal dimer. Such a high DOS should be related to the anharmonicity of the potential energy surface near the barrier of the GSDPT reaction. To get more information, an N-D deuteration effect is examined. In the present experiment, five deuterated dimers are identified by visible-visible or IR-visible population labeling spectroscopy. The IR band pattern of the NH-ND dimer is very different from that of the NH-NH dimer. Among several N-D deuteration effects, a change in a condition between the inter- or intramolecular vibrational energy flows due to the single N-D deuteration is considered to be important.

Algebraic analysis of bent-from-linear transition intensities: the vibronically resolved emission spectrum of methinophosphide (HCP)

Abstract Emission spectra obtained from bulk-gas methinophosphide (HCP) have been interpreted by means of a Lie algebraic theory that describes explicit non-rigidity of the molecular framework. Fluorescence accompanying selective excitation of A 1 A ″– X 1 Σ + vibronic bands was dispersed under moderate resolution, with substantial activity of the ν2 H–C–P bending mode reflecting the bent-from-linear nature of the A ← X transition. Aside from furnishing an economical parameterization for energy level patterns, the algebraic treatment permits Franck–Condon intensities to be evaluated quantitatively. The equilibrium structure deduced for the A 1 A ″ state is in good accord with quantum chemistry calculations except for substantially less than predicted elongation of the C–P bond upon electron promotion.

The vibrational spectrum and molecular constants of silicon dihydride SiH2 in the ground electronic state

The infrared spectrum of SiH2 was observed in the 5 μm region by using diode laser spectroscopy, and the three bands ν1, 2ν2, and ν3 were recorded and assigned. A separate analysis of the three bands yielded the band origins, 1995.9280, 1987.6938, and 1992.816 cm−1, respectively, thereby establishing that ν1 is higher than 2ν2, contrary to the result of a previous matrix study. The anharmonic resonances, both ν1/2ν2 Fermi and 2ν1/2ν3 Darling–Dennison, which were observed in the laser induced fluorescence spectra previously reported, were reanalyzed by removing the condition ν1<2ν2, leading to a much improved set of vibrational parameters. The harmonic frequencies of SiH2 were then obtained to be ω1=2076.55, ω2=1020.49, and ω3=2065.65 cm−1. Molecular constants of SiH2 in the ground vibronic state were reexamined in light of the present as well as other spectroscopic results recently obtained on the molecule.

First observation of a dihydrogen bond involving the Si–H group in phenol-diethylmethylsilane clusters by infrared-ultraviolet double-resonance spectroscopy

We have experimentally identified a dihydrogen bond involving the Si-H group in phenol-diethylmethylsilane (DEMS) clusters for the first time by IR-UV double-resonance spectroscopy. Vibrational shifts to lower frequency of 21-29 cm(-1) were found for the OH stretching vibration of three isomers of the phenol-DEMS clusters. Spectral simulations based on the MP2 calculations also support our observation. In addition to these clusters, dihydrogen bonds were also observed in the phenol-H(2)O-DEMS and (phenol)(2)-DEMS clusters, which exhibited much stronger interactions than the phenol-DEMS clusters.

First observation of the B̃A11 state of SiH2 and SiD2 radicals by optical-optical double resonance spectroscopy

The B1A1 state of SiH2 and SiD2 was observed by the optical-optical double resonance technique for the first time. The electronic band origin of the B state of SiD2 was determined to be 27 214.11 cm(-1). A very clear exclusive behavior depending on the even/odd value of the bending vibrational quantum number was observed in the spectra, representing a quasilinear behavior of the B state. The barrier height to linearity was estimated to be approximately 125 cm(-1) by the quasilinear analysis of the bending vibrational level structure of SiD2.