Spectroscopic and Theoretical Studies on Conformational Stability of Benzyl Methyl Ether.

Conformer-selected electronic and vibrational spectra of benzyl methyl ether and its terminal methyl group-substituted derivative in a supersonic jet have been measured using ultraviolet (UV)-UV hole burning and fluorescence-detected infrared spectroscopy to investigate the conformational stability of flexible molecules. Various quantum chemical calculations as well as experimental observations reveal the coexistence of three conformers with different CCOC dihedral angles and side-chain orientations relative to the benzene ring plane. Vibrational analysis in the excited state with time-dependent density functional theory and IR simulations containing anharmonic coupling sufficiently reproduce the experimental results, suggesting that these three conformers can be distinguished into one gauche-conformer and two trans-ones with respect to the CCOC dihedral angle. We also observe that the gauche conformer exhibits higher-frequency CH2 modes. The natural bond orbital analysis indicates that this phenomenon is attributed to the electron delocalization from the non-bonding orbitals and the C-O orbitals associated with the neighboring oxygen atom, which leads to a conformer dependence of the methylene C-H bond strength.

Isomer Stability Dependence of Hydrogen-Bonded Benzoxazole Clusters on Solvent Molecules.

The structures of hydrogen-bonded benzoxazole clusters with methanol and ammonia, BO-(CH3OH)n (n = 1-3) and BO-(NH3)n (n = 1, 2), in a supersonic jet have been investigated by measuring the S1-S0 electronic spectra and isomer-selected vibrational spectra with the aid of quantum chemical calculations. Similar to BO-(H2O)1, two isomers of BO-(NH3)1 were observed, which form two types of hydrogen bond networks starting from the CH bond at the 2-/7-position to the nitrogen atom of BO (C2HN/C7HN). The relative stability of these isomers strongly depends on solvent molecules. Natural bond orbital analysis reveals that the OH···N hydrogen bond is dominant in BO-(H2O)1 and that intermolecular interaction between the CH group and the nitrogen atom of ammonia, especially C2H···N, is significantly enhanced, resulting in a more stable C2HN isomer. Symmetry-adapted perturbation theory calculations indicate that the dispersion interaction between the methyl group of methanol and π electron cloud on the BO ring is responsible for the extreme stability of the C7HN of BO-(CH3OH)1. Furthermore, using time-dependent density functional theory calculations, the isomer tendency of the electronic transition shifts from the monomer origin is reproduced and it is proposed that the significant blue shift in C2HN is due to the shortened C2H bond length upon electronic excitation.

Excited-State Dynamics Affected by Switching of a Hydrogen-Bond Network in Hydrated Aminopyrazine Clusters.

The cluster structures of hydrated aminopyrazines, APz-(H2O)n=2-4, in supersonic jets have been investigated measuring the size-selected electronic and vibrational spectra and determined with the aid of quantum chemical calculations. The APz-(H2O)2 structure is assigned as a cyclic N1 type where a homodromic hydrogen-bond chain starts from the amino group and ends at the 1-position nitrogen atom of the pyrazine moiety, corresponding to 2-aminopyridine-(H2O)2. On the other hand, APz-(H2O)n=3,4 has a linear hydrogen-bond network ending at the 4-position one (N4), which resembles 3-aminopyridine-(H2O)n=3,4. The hydrogen-bond network switching from the N1 type to the N4 one provides the accompanying red shifts of the S1-S0 electronic transition that are entirely consistent with those of the corresponding 2-aminopyridine and 3-aminopyridine clusters and also shows the drastically strengthened fluorescence intensity of origin bands in the electronic spectrum. The significant change in the excited-state dynamics is explored by the fluorescence lifetime measurement and the time-dependent density functional theory (TD-DFT) calculation. It is suggested that the drastic elongation of fluorescence lifetimes is due to the change in the electronic structure of the first excited state from nπ* to ππ*, resulting in the decreasing spin-orbit coupling to T1 (ππ*).

Drastic Change in Electronic Transition upon Hydrogen Bond Network Switching in 3-Aminopyridine-(H2O) n Clusters.

The hydration structures of 3-aminopyridine (3AP)-(H2O) n ( n = 2-4) in supersonic jets have been investigated by measuring the electronic and vibrational spectra with the aid of quantum chemical calculations. The S1-S0 electronic transition of 3AP-(H2O)2 is observed at a slightly red-shifted position from 3AP-(H2O)1, while further hydration induces drastic red shifts and complicated vibrational structures. We assign the cluster structures of 3AP-(H2O)2 as a cyclic structure composed of the homodromic hydrogen bond (H-bond) chain connecting the pyridyl CH bond acting as the proton donor toward a pyridyl nitrogen acceptor. For 3AP-(H2O) n ( n = 3, 4), on the other hand, the initial donor site in the H-bond network changes from a pyridyl CH group to an amino group. The observed red shift derived from H-bond network switching can be reproduced very well with the S1-S0 origin band estimation obtained by applying geometry optimization and subsequent harmonic vibrational analysis of (TD-)DFT calculations to each electronic state of the isomer structure. It is suggested that the drastic red shift of the electronic transition upon H-bond network switching is due to a much more stabilized "quinoid-like" structure in the ππ* state by the H-bond formation of an amino group.

Comparable strength of OH-O versus OH-π hydrogen bonds in hydrogen-bonded 2,3-benzofuran clusters with water and methanol.

Two different types of hydrogen bond, which are classified into a familiar OH-O and a relatively weak OH-π one, have been compared in the 1:1 hydrogen-bonded 2,3-benzofuran clusters with water and methanol molecules. By applying fluorescence-detected infrared spectroscopy and dispersed fluorescence spectroscopy, two isomers having different types of hydrogen bonds are distinguished. From the calculated stabilization energy as well as the frequency shift of the OH stretching vibration in each cluster, these two isomers are almost equally stable, although that of OH-π type is usually thought to be relatively weak. It is suggested that the origin of the weak OH-O hydrogen bond is derived from the lower availability for a hydrogen bond acceptor on the oxygen atom of a heteroaromatic ring, which is attributed to the larger furan aromaticity.

Solvation effect on the NH stretching vibrations of solvated aminopyrazine, 2-aminopyridine, and 3-aminopyridine clusters.

The vibrational spectra of the hydrated and methanol-solvated aminopyrazine, 2-aminopyridine and 3-aminopyridine in supersonic jets have been measured in terms of IR-UV double-resonance spectroscopy. Comparing the IR spectrum of aminopyrazine with those of 2-aminopyridine and 3-aminopyridine clusters, we determine the solvation structure of aminopyrazine to be a similar cyclic structure as hydrated 2-aminopyridine clusters [Wu, et al., Phys. Chem. Chem. Phys. 2004, 6, 515]. In the case of monohydrated aminopyrazine cluster, one of the normal modes composed of the hydrogen-bonded OH and NH stretching local modes gives the anomalously weak IR intensity, which is ascribed to the cancellation of the dipole moment change between the OH and NH stretching local modes. The solvated 3-aminopyridine clusters forms the hydrogen-bond between the pyridyl nitrogen atom and the OH group, but the amino group is indirectly affected to induce slight blue shift of the NH(2) stretches. This phenomenon is explained by inductive effect where the electron withdrawing from the amino group upon the solvation results in a "quinoid-like" structure of the amino group.

Conformation of 2,2,2-trifluoroethanol and the solvation structure of its 2-fluoropyridine clusters.

The conformation of 2,2,2-trifluoroethanol (TFE) in the 2-fluoropyridine-(TFE)(m)-(H(2)O)(n) clusters in a supersonic jet has been investigated with fluorescence-detected infrared spectroscopy and quantum chemical calculations. It is common to the observed clusters that they form chain structures containing the weak interaction of the pyridyl CH with the fluorine or oxygen atom in the terminal TFE. The detectable conformation of TFE is gauche only even in the case of the existence of the strong base such as 2-fluoropyridine. This result is explained by the change in hyperconjugation among several dominant orbitals. The preference of the terminal TFE in the mixed clusters with TFE and water solvents is observed, which is ascribed to the stronger cooperative effect of TFE than water.

Radio frequency magnetic field effects on molecular dynamics and iron uptake in cage proteins.

The protein ferritin has a natural ferrihydrite nanoparticle that is superparamagnetic at room temperature. For native horse spleen ferritin, we measure the low field magnetic susceptibility of the nanoparticle as 2.2 x 10(-6) m(3) kg(-1) and its Néel relaxation time at about 10(-10) s. Superparamagnetic nanoparticles increase their internal energy when exposed to radio frequency magnetic fields due to the lag between magnetization and applied field. The energy is dissipated to the surrounding peptidic cage, altering the molecular dynamics and functioning of the protein. This leads to an increased population of low energy vibrational states under a magnetic field of 30 microT at 1 MHz, as measured via Raman spectroscopy. After 2 h of exposure, the proteins have a reduced iron intake rate of about 20%. Our results open a new path for the study of non-thermal bioeffects of radio frequency magnetic fields at the molecular scale.

IR spectroscopy of hydrogen-bonded 2-fluoropyridine-methanol clusters.

The electronic and infrared spectra of 2-fluoropyridine-methanol clusters were observed in a supersonic free jet. The structure of hydrogen-bonded clusters of 2-fluoropyridine with methanol was studied on the basis of the molecular orbital calculations. The IR spectra of 2-fluoropyridine-(CH3OH)n(n = 1-3) clusters were observed with a fluorescence-detected infrared depletion (FDIR) technique in the OH and CH stretching vibrational regions. The structures of the clusters are similar to those observed for 2-fluoropyridine-(H2O)n (n = 1-3) clusters. The existence of weak hydrogen bond interaction through aromatic hydrogen was observed in the IR spectra. The theoretical calculation also supports the result. The vibrational frequencies of CH bonds in CH3 group are affected by hydrogen bond formation although these bonds do not directly relate to the hydrogen bond interaction. The B3LYP/6-311 ++G(d,p) calculations reproduce well the vibrational frequency of the hydrogen-bonded OH stretching vibrations. However, the calculated frequency of CH stretching vibration could not reproduce the IR spectra because of anharmonic interaction with closely lying overtone or combination bands for nu3 and nu9 vibrations. The vibrational shift of nu2 vibration is reproduced well with molecular orbital calculations. The calculation also shows that the frequency shift of nu2 vibration is closely related to the CH bond length at the trans position against the OH bond in hydrogen-bonded methanol.

Infrared spectroscopy of hydrogen-bonded 2-fluoropyridine-water clusters in supersonic jets.

The hydrogen-bonded clusters of 2-fluoropyridine with water were studied experimentally in a supersonic free jet and analyzed with molecular orbital calculations. The IR spectra of 2-fluoropyridine-(H2O)(n) (n = 1 to 3) clusters were observed with a fluorescence detected infrared depletion (FDIR) technique in the OH and CH stretching vibrational regions. The frequencies of OH stretching vibrations show that water molecules bond to the nitrogen atom of 2-fluoropyridine in the clusters. The hydrogen-bond formation between aromatic CH and O was evidenced in the 1:2 and 1:3 clusters from the experimental and calculated results. The overtone vibrations of the OH bending mode in hydrogen-bonded water molecules appear in the IR spectra, and these frequencies become higher with the increase of the number of water molecules in the clusters. The band structure of the IR spectra in the CH stretching region changes depending on the number of coordinating water molecules.

Synthesis and reaction of alpha-dithiolactone.

Treatment of di-tert-butylthioketene S-oxide (5 a) with Lawesson reagent at room temperature resulted in the formation of 3,3-di-tert-butylthiirane-2-thione (4 a) in high yield. The oxidation of 4 a with mCPBA (mCPBA=m-chloroperbenzioc acid) gave 3,3-di-tert-butylthiirane-2-thione S-oxide (6) almost quantitatively. The reactions of 4 a with dimethyl acetylenedicarboxylate (DMAD) and benzyne afforded dimethyl 2-(2,2,4,4-tetramethylpentan-3-ylidene)-1,3-dithiole-4,5-dicarboxylate (13) and 2-(2,2,4,4-tetramethylpentan-3-ylidene)benzo[d][1,3]dithiole (15), respectively, in high yields, suggesting that 4 a is an excellent 1,3-dipole. The reaction of 4 a with ethylenebis(triphenylphosphine)platinum (16) gave dithiolato-platinum complex (22) in high yield. The structure of 22 was determined by X-ray crystallographic analysis.

Electronic spectra of hydrogen-bonded 2-fluoropyridine clusters with water in a supersonic free jet.

Fluorescence excitation, multiphoton ionization, and dispersed fluorescence spectra of bare and hydrogen-bonded 2-fluoropyridine with water were measured in a supersonic free jet. For bare 2-fluoropyridine, fluorescence quantum yield decrease in the higher vibronic levels was observed even under collision-free conditions. The inter-system crossing channel was probed experimentally by two color R2PI and found to be negligible. The non-radiative relaxation process of 2-fluoropyridine is mainly governed by the relaxation to the electronic ground state. Electronic spectra of 2-fluoropyridine-(water)(n) (n=1 approximately 3) were also obtained. The hydrogen bond formation with water increases the quantum yield in the higher vibronic levels. Rather low frequency vibrations were observed in the hole burning spectrum of bare 2-fluoropyridine; however, these vibronic bands disappeared with the hydrogen bond formation with water. The appearance of low frequency vibronic bands observed for bare 2-fluoropyridine is ascribed to the existence of closely lying (n,pi) state.

Synthesis of isolable thiirane-2-thione (alpha-dithiolactone) from thioketene S-oxide.

Reaction of di-tert-butyl thioketene S-oxide with Lawesson reagent gave 3,3-di-tert-butylthiirane-2-thione in 88% yield. Oxidation of thiirane-2-thione with m-chloroperbenzoic acid afforded 3,3-di-tert-butylthiirane-2-thione S-oxide. The reaction of thiirane-thione was described.