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Laser-induced fluorescence (LIF) spectroscopy and fluorescence-detected ultraviolet (UV)-UV and infrared (IR)-UV double-resonance spectroscopy are used to study the structure of jet-cooled L-tyrosine (L-Tyr). The comparison of the results of L-Tyr and L-phenylalanine have shown that conformers of L-Tyr existing under jet-cooled condition have side-chain conformers similar to conformers B, D, E, and X of L-phenylalanine.

The study observes NH stretching vibrations of jet-cooled aniline and its various derivatives in the neutral and cationic ground states. The results reveal that the intramolecular hydrogen bond formation was found for o-cyanoaniline in both the neutral and cationic ground states based on the infrared spectra of deuterated isotopomers resulting in hydrogen bond strength, which is remarkably enhanced upon ionization.

Infrared photodissociation spectra of n-propylbenzene (n-PB)-Ar cluster cations are presented in the CH stretching vibrational region. Spectral analysis suggests that ionization of (n-PB)-Ar cluster cations show delocalization of positive charge of aromatic ring into the alkyl chain.

Research is done on infrared spectra of 2-hydroxypyridine (2HP) and its hydrogen-bonded clusters with water and methanol have been observed by IR-UV double-resonance spectroscopy. In conclusion, the cluster structures were examined using ab initio molecular orbital measurements.

Research was done on the vibrational relaxation of the OH/OD stretching mode of phenol and its cluster with Ar or H2O which had been examined directly by nanosecond IR--UV pump--probe spectroscopy. In conclusion, the experiment on the phenol--Ar and phenol--H2O reveal the OH stretching level, which showed a fast single exponential decay curve, indicating a transformation from intermediate coupling to the statistical limit upon the cluster formation.

Methyl salicylate (MS) and its hydrogen-bonded clusters with several solvents in the OH stretch region are studied using infrared spectroscopy. Results show that each cluster holds the intramolecular hydrogen bond although the bond is perturbed by the intermolecular hydrogen-bond formation with the solvent molecules. Intramolecular bond breakdown with microsolvation was not observed in small size MS clusters while complete tranformation from intra- to intermolecular hydrogen was found in tropolone.

Geometric structures and excited state proton dislocation of size-selected salicylic acid clusters with water were studied by using laser spectroscopic techniques. The result showed that infrared (IR) spectra in S1 and dispersed fluorescence spectra of the water do not remarkably affect the excited state proton dislocation in contrast to the significant suppression of the proton dislocation in the homodimers.

Infrared (IR) spectra of size-selected benzene-water cluster cations ([C6H6-(H2O)n](super +), n = 1-23) were measured in the OH and CH stretch regions to investigate the cluster structures, especially at large sizes. It was found that in the large sized clusters, while the protonated water moiety forms the exclusive hydrogen bond network, the phenyl radical is finally pushed out to the exterior of the network, reflecting the microscopic hydrophobicity of the aromatic ring.

Infra-red spectroscopy for the CH-stretching vibration of jet-cooled clusters of acetylene with various aromatic, phenylsilane, ethylbenzene, naphthalene, and anthracene in both the electronic ground (S0) and first excites (S1) was carried out. The acetylenic stretching vibrations of the clusters were observed in S0 and S1 upon which, the acetylenic CH stretch band showed a distinct low-frequency shift, which is the consequence of the activated CH/pi interaction in these clusters.

An ultraviolet (UV)-infrared (IR) double resonance spectroscopy with related fluorescence detection is applied to the observation to the IR spectrum of aniline in the first electronically excited (S(sub 1)) state. On examining the IR spectra for several deuterium substituted species, it is found that NH2 stretching vibrations of the S(sub 1) state are located at 3000 cm(super -1), representing extremely large frequency decrease upon electronic excitation.

OH stretching vibrations region indicated infrared spectra of phenol-X (X = C6H6, C2H4, and C2H2) clusters in the neutral and cationic ground states. A small low-frequency shift of the OH vibration of the phenol site in all the clusters represented the characteristic feature for their Pi-hydrogen-bonded structures.

The structures and the excited-state proton-transfer reaction in supersonic jets are studied. The results present a direct evidence for the enol bridge form 7HQ-(CH3OH)3 to be a precursor for the proton-transfer reaction in solution.

Research is done on the OH and CH stretching vibrations of fluorene-(H2O)(sub n=1,2) clusters, which have been observed by IR-UV and stimulated Raman-UV double resonance spectroscopy in supersonic jets. Finally, from the observed spectra and the ab initio calculations at HF/6-31G and H/6-31G(d,p) levels, it was concluded that the n = 1 and 2 clusters have similar structures in which the water monomer or the dimer locates above the plane of fluorene.

Autoionization-detected infrared (ADIR) spectroscopy is applied in order to observe OH stretching vibrations of jet-cooled 1-and 2-naphthol cations. The findings indicate that perturbations from the Ar atom to the hydroxyl group of naphthol are negligible in both the neutral and cationic ground states.

We measure UV photodissociation (UVPD)spectra of cold benzo-15-crown-5(B15C5) and benzo-18-crown-6 (B18C6) complexes with divalent ions(M2+= Ca2+, Sr2+, Ba2+, and Mn2+), solvated with an H2O or a CH3OH molecule: M2+·B15C5·H2O,M2+·B15C5·CH3OH, M2+·B18C6·H2O, and M2+·B18C6·CH3OH. Allthe species show a number of sharp vibronic bands in the 36600–37600cm–1region, which can be attributed to electronictransitions of the B18C6 or B15C5 component. Conformer-specific IRspectra of these complexes are also obtained by IR-UV double-resonancespectroscopy in the OH stretching region. All the IR-UV spectra ofthe H2O complexes show IR bands at ∼3610 and ∼3690cm–1; these bands can be assigned to the symmetricand asymmetric OH stretching vibrations of the H2O component.The CH3OH complexes also show the stretching vibrationof the OH group at ∼3630 cm–1. The H2O and the CH3OH components are directly bondedto the M2+ion through the M2+···Obond in all the complexes, but a small difference in the conformationresults in a noticeable difference in the OH stretching frequency,which enables us to determine the number of conformers. For Ca2+, Sr2+, and Mn2+, the number of conformersfor the B18C6 complexes is in the range of 2–5, which is clearlylarger than complexes with B15C5 (1 or 2). However, for Ba2+the number of conformers with B18C6 (1 or 2) is almost the sameas that with B15C5. This is probably because the Ba2+ionis too large to be located in the cavity center of either B15C5 andB18C6, which provides an open site at the Ba2+ion suitablefor solvation with H2O or CH3OH. The more conformationsa complex can take, the more entropically favored it is at nonzerotemperatures. Hence, the larger number of conformations suggests higherstability of the complexes under solvated conditions, leading to ahigher degree of ion encapsulation in solution. [ABSTRACT FROM AUTHOR]

The IR spectra of benzoic acid (BA), (BA)(H 2 O) n and (BA) 2 (H 2 O) n ( n = 1, 2) clusters, and their ring-deuterated isotopologues in the 2800-3750 cm -1 region were measured with IR-vacuum ultraviolet spectroscopy under the jet-cooled condition. For (BA)(H 2 O) and (BA)(H 2 O) 2 , only a single isomer was observed for each species, whereas for (BA) 2 (H 2 O) and (BA) 2 (H 2 O) 2 , more than one isomers were present. The observed IR spectra were very complex and showed similar structures between (BA) m (H 2 O) n and their ring-deuterated isotopologues (BA- d 5 ) m (H 2 O) n for specific values of m and n . The anharmonic analysis based on the vibrational second-order perturbation theory indicated that the complexity of the IR spectra in these clusters was due to the appearance of many bands of (i) the overtone and combination modes involving the O-H bend of H 2 O and the in-plane C-O-H bends and the C═O stretch of BA, and (ii) the combination modes involving the hydrogen-bonded O-H stretch and low-frequency intermolecular vibrations, with considerable intensities.

The structures of a pyrazine dimer (pyrazine) 2 and (pyrazine)(benzene) hetero-dimer cooled in a supersonic beam were investigated by the measurement of the infrared spectra in the C-H stretching region with infrared-vacuum ultraviolet (IR-VUV) spectroscopy and quantum-chemical calculations. The stabilization energy calculation at the CCSD(T)/aug-cc-pVTZ level of theory predicted three isomers for (pyrazine) 2 and three for (pyrazine)(benzene) with energy within 6 kJ/mol. Among them, the cross-displaced π-π stacked structure is the most stable in both dimers. In the observed IR spectra, both dimers exhibited two intense bands near 3065 cm -1 , with intervals of 8 cm -1 in (pyrazine) 2 and 11 cm -1 in (pyrazine)(benzene), while only one band appeared in the monomer. For (pyrazine)(benzene), we also measured the IR spectrum of (pyrazine)(benzene- d 6 ), where the interval of the two bands was unchanged. The analysis of the observed IR spectra with anharmonic calculations suggested the coexistence of three isomers of (pyrazine) 2 and (pyrazine)(benzene) in a supersonic jet. For (pyrazine) 2 , the two isomers which were previously assigned to the H-bonded planar and the π-π stacked structures respectively were reassigned to the cross-displaced π-π stacked and T-shaped structures, respectively. In addition, the quantum chemical calculation and IR-VUV spectral measurement suggested the coexistence of the H-bonded planar isomer in the jet. For (pyrazine)(benzene), the IR spectrum of the (pyrazine) site showed a similar spectral pattern to that of (pyrazine) 2 , especially the split at ∼3065 cm -1 . However, the anharmonic analysis suggested that they are assigned to the different vibrational motions of (pyrazine). The anharmonic vibrational analysis is essential to associate the observed IR spectra with the correct structures of the dimer.

The infrared (IR) spectra of the O-H stretching vibrations of pyridine-water clusters (Pyd) m (H 2 O) n , with m , n = 1-4, have been investigated with infrared-vacuum ultraviolet (VUV) spectroscopy under a jet-cooled condition. The time-of-flight mass spectrum of (Pyd) m (H 2 O) n + by VUV ionization at ∼9 eV showed an unusual intensity pattern with very weak ion signals for m = 1 and 2 and stronger signals for m ≥ 3. This unusual mass pattern was explained by a drastic structural change of (Pyd) m (H 2 O) n upon the VUV ionization, which was followed by the elimination of water molecules. Among the recorded IR spectra, only one spectrum monitored, (Pyd) 2 + cation, showed a well-resolved structure. The spectrum was analyzed by comparing with the simulated ones of possible stable isomers of (Pyd) 2 (H 2 O) n , which were obtained with quantum-chemical calculations. Most of the calculated (Pyd) 2 (H 2 O) n clusters had the characteristic structure in which H 2 O or (H 2 O) 2 forms a hydrogen-bonded bridge between two pyridines to form the π-stacked (Pyd) 2 , and an additional H 2 O molecule(s) extends the H-bonded network. The π-stacked (Pyd) 2 (H 2 O) n moiety is very stable and is thought to exist as a local structure in a pyridine/water mixed solution. The Fermi resonance between the O-H stretch fundamentals and the overtones of the O-H bending vibrations in (Pyd) m (H 2 O) n was found to be less pronounced in the case of (Pyd) m (NH 3 ) n studied previously.

The conformation and electronic structure of dibenzo-24-crown-8 (DB24C8) complexes with K + ion were examined by ion mobility-mass spectrometry (IM-MS), ultraviolet (UV) photodissociation (UVPD) spectroscopy in the gas phase, and fluorescence spectroscopy in solution. Three structural isomers of DB24C8 (SymDB24C8, Asym1DB24C8, and Asym2DB24C8) in which the relative positions of the two benzene rings were different from each other were investigated. The IM-MS results at 86 K revealed a clear separation of two sets of conformers for the K + (SymDB24C8) and K + (Asym1DB24C8) complexes whereas the K + (Asym2DB24C8) complex revealed only one set. The two sets of conformers were attributed to the open and closed forms in which the benzene-benzene distances in the complexes were long (>6 Å) and short (<6 Å), respectively. IM-MS at 300 K could not separate the two conformer sets of the K + (SymDB24C8) complex because the interconversion between the open and closed conformations occurred at 300 K and not at 86 K. The crown cavity of DB24C8 was wrapped around the K + ion in the complex, although the IM-MS results availed direct evidence of rapid cavity deformation and the reconstruction of stable conformers at 300 K. The UVPD spectra of the K + (SymDB24C8) and K + (Asym1DB24C8) complexes at ∼10 K displayed broad features that were accompanied by a few sharp vibronic bands, which were attributable to the coexistence of multiple conformers. The fluorescence spectra obtained in a methanol solution suggested that the intramolecular excimer was formed only in K + (SymDB24C8) among the three complexes because only SymDB24C8 could possibly assume a parallel configuration between the two benzene rings upon K + encapsulation. The encapsulation methods for K + ion (the "wraparound" arrangement) are similar in the three structural isomers of DB24C8, although the difference in the relative positions of the two benzene rings affected the overall cross-section. This study demonstrated that temperature-controlled IM-MS coupled with the introduction of appropriate bulky groups, such as aromatic rings to host molecules, could reveal the dynamic aspects of encapsulation in host-guest systems.

We performed UV spectroscopy for p -coumaric acid ( p CA), ferulic acid (FA), and caffeic acid (CafA) under jet-cooled gas-phase conditions by using a laser-ablation source. These molecules showed the S 1 ( 1 ππ*)-S 0 absorption in the 31 500-33 500 cm -1 region. Both p CA and FA exhibited sharp vibronic bands, while CafA showed only a broad feature. The decay time profile of the 1 ππ* state was measured by picosecond pump-probe spectroscopy, and the transient state produced through the nonradiative decay (NRD) from 1 ππ* and its time profile were measured by nanosecond UV-deep UV pump-probe spectroscopy. The transient state was observed for p CA and FA and assigned to the T 1 state, and we concluded that the NRD process of 1 ππ* is S 1 ( 1 ππ*) → 1 nπ* → T 1 ( 3 ππ*), similar to those of methyl cinnamate and para -substituted cinnamates such as p -hydroxy and p -methoxy methyl cinnamate. On the other hand, the transient T 1 state was not detected in CafA, and its NRD route is suggested to be S 1 ( 1 ππ*) → 1 πσ* → H atom elimination, similar to those of phenol and catechol. The effect of a hydrogen bond on the electronic state and NRD process was investigated, and it was found that the H-bonding lowers the 1 ππ* energy and suppresses the NRD process for all the species.

The title compound, 2-ethylhexyl-4-methoxycinnamate (2EH4MC), is known as a typical ingredient of sunscreen cosmetics that effectively converts the absorbed UV-B light to thermal energy. This energy conversion process includes the nonradiative decay (NRD): trans - cis isomerization and finally going back to the original structure with a release of thermal energy. In this study, we performed UV spectroscopy for jet-cooled 2EH4MC to investigate the electronic/geometrical structures as well as the NRD mechanism. Laser-induced-fluorescence (LIF) spectroscopy gave the well-resolved vibronic structure of the S 1 -S 0 transition; UV-UV hole-burning (HB) spectroscopy and density functional theory (DFT) calculations revealed the presence of syn and anti isomers, where the methoxy (-OCH 3 ) groups orient in opposite directions to each other. Picosecond UV-UV pump-probe spectroscopy revealed the NRD process from the excited singlet (S 1 ( 1 ππ*)) state occurs at a rate constant of ∼10 10 -10 11 s -1 , attributed to internal conversion (IC) to the 1 nπ* state. Nanosecond UV-deep UV (DUV) pump-probe spectroscopy identified a transient triplet (T 1 ( 3 ππ*)) state, whose energy (from S 0 ) and lifetime are 18 400 cm -1 and 20 ns, respectively. These results demonstrate that the photoisomerization of 2EH4MC includes multistep internal conversions and intersystem crossings, described as "S 1 ( trans , 1 ππ*) → 1 nπ* → T 1 ( 3 ππ*) → S 0 ( cis )".

The UV photodissociation (UVPD) spectra of Ag + complexes with benzo-18-crown-6 (B18C6), dibenzo-18-crown-6 (DB18C6), and dibenzo-15-crown-5 (DB15C5) [Ag + (B18C6), Ag + (DB18C6), and Ag + (DB15C5)] are observed under cold gas-phase conditions. Ag + (B18C6) and Ag + (DB18C6) show sharp vibronic bands in the 36000-37200 cm -1 region, while the UVPD spectrum of Ag + (DB15C5) is very broad. These UV bands are assigned to the π-π* transition, which is localized on the B18C6, DB18C6, and DB15C5 part of the complexes. Quantum chemical calculations suggest that the broad UV feature of Ag + (DB15C5) can be attributed to the short lifetimes of optically excited ππ* states due to internal conversion (IC) to low-lying excited states that are present only for this complex. The appearance of the π-π* transition in the same UV region as that of the neutral crown ethers and their complexes with alkali metal ions indicates that the positive charge is localized on the Ag atom in these complexes. However, the fragment ions produced after UV absorption are B18C6 + , DB18C6 + , and DB15C5 + radical ions, indicating that they are produced via charge transfer (CT) between the Ag + ion and benzo-crown ethers. The CT during fragmentation is attributed to the higher ionization energy of Ag atom when compared to the benzo-crown ethers. In the complexes, the Ag + ion is effectively encapsulated by the crown cavity of the benzo-crown ethers without transferring the positive charge from Ag + to the crown. However, UV excitation of the Ag + (B18C6), Ag + (DB18C6), and Ag + (DB15C5) complexes can reduce the Ag + ion and produce a Ag atom with high efficiency in the gas phase.

Ultraviolet photodissociation (UVPD) and IR-UV double-resonance spectroscopy are performed for bare and microhydrated complexes of Mn 2+ (benzo-15-crown-5), Mn 2+ (B15C5)(H 2 O) n ( n = 0-2), under cold (∼10 K) gas-phase conditions. Density functional theory (DFT) calculations are also carried out to derive information on the geometric and electronic structures of the complexes from the experimental results. The n = 0 complex shows broad features in the UVPD spectrum, whereas the UV spectra of the n = 1 and 2 complexes exhibit sharp vibronic bands. The IR-UV and DFT results suggest that there is only one isomer each for the n = 1 and 2 complexes in which H 2 O molecules are directly attached to the Mn 2+ ion through Mn 2+ ···OH 2 bonds with no intermolecular bond between the water molecules. Time-dependent DFT calculations suggest that the π-π* transition of the B15C5 part is highly mixed with the "ligand to metal charge transfer" transition in the n = 0 complex, which can result in broad features in the UVPD spectrum. In contrast, attachment of H 2 O molecules to Mn 2+ (B15C5) suppresses the mixing, providing sharp vibronic bands assignable to the π-π* transition for the n = 1 and 2 complexes. These results indicate that the electronic structure and transition of benzo-crown ether complexes with transition metals are strongly affected by solvation.

In this Article, we examine the hydration structure of dibenzo-18-crown-6 (DB18C6) complexes with K + , Rb + , and Cs + ion in the gas phase. We measure well-resolved UV photodissociation (UVPD) spectra of K + ·DB18C6·(H 2 O) n , Rb + ·DB18C6·(H 2 O) n , and Cs + ·DB18C6·(H 2 O) n ( n = 1-8) complexes in a cold, 22-pole ion trap. We also measure IR-UV double-resonance spectra of the Rb + ·DB18C6·(H 2 O) 1-5 and the Cs + ·DB18C6·(H 2 O) 3 complexes. The structure of the hydrated complexes is determined or tentatively proposed on the basis of the UV and IR spectra with the aid of quantum chemical calculations. Bare complexes (K + ·DB18C6, Rb + ·DB18C6, and Cs + ·DB18C6) have a similar boat-type conformation, but the distance between the metal ions and the DB18C6 cavity increases with increasing ion size from K + to Cs + . Although the structural difference of the bare complexes is small, it highly affects the manner in which each is hydrated. For the hydrated K + ·DB18C6 complexes, water molecules bind on both sides (top and bottom) of the boat-type K + ·DB18C6 conformer, while hydration occurs only on top of the Rb + ·DB18C6 and Cs + ·DB18C6 complexes. On the basis of our analysis of the hydration manner of the gas-phase complexes, we propose that, for Rb + ·DB18C6 and Cs + ·DB18C6 complexes in aqueous solution, water molecules will preferentially bind on top of the boat conformers because of the displaced position of the metal ions relative to DB18C6. In contrast, the K + ·DB18C6 complex can accept H 2 O molecules on both sides of the boat conformation. We also propose that the characteristic solvation manner of the K + ·DB18C6 complex will contribute entropically to its high stability and thus to preferential capture of K + ion by DB18C6 in solution.

We measure UV photodissociation (UVPD) and UV-UV hole-burning (HB) spectra of dibenzo-15-crown-5 (DB15C5) complexes with alkali metal ions, M + ·DB15C5 (M = Li, Na, K, Rb, and Cs), under cold (∼10 K) conditions in the gas phase. The UV-UV HB spectra of the M + ·DB15C5 (M = K, Rb, and Cs) complexes indicate that there is one dominant conformation for each complex except the Na + ·DB15C5 complex, which has two conformers with a comparable abundance ratio. It was previously reported that the M + ·(benzo-15-crown-5) (M + ·B15C5, M = K, Rb, and Cs) complexes each have three conformers. Thus, the attachment of one additional benzene ring to the crown cavity of benzo-15-crown-5 reduces conformational flexibility, giving one dominant conformation for the M + ·DB15C5 (M = K, Rb, and Cs) complexes. In the UVPD spectra of the K + ·DB15C5, Rb + ·DB15C5, and Cs + ·DB15C5 complexes, the S 1 -S 0 and S 2 -S 0 transitions are observed independently at different positions with different vibronic structures. The spectral features are substantially different from those of the K + ·(dibenzo-18-crown-6) (K + ·DB18C6) complex, which belongs to the C 2v point group and exhibits exciton splitting with an interval of 2.7 cm -1 . The experimental and theoretical results suggest that in the M + ·DB15C5 complexes the two benzene rings are not symmetrically equivalent with each other and the S 1 -S 0 and S 2 -S 0 electronic excitations are almost localized in one of the benzene rings. The electronic interaction energy between the two benzene chromophores is compared between the K + ·DB15C5 and K + ·DB18C6 complexes by quantum chemical calculations. The interaction energy of the K + ·DB15C5 complex is estimated to be less than half of that of the K + ·DB18C6 complex (∼30 cm -1 ) due to less suitable relative angles between the transition dipole moments of the two benzene chromophores in K + ·DB15C5.

We measure UV photodissociation (UVPD) spectra of benzo-12-crown-4 (B12C4) complexes with alkali metal ions, M(+)·B12C4 (M = Li, Na, K, Rb, and Cs), in the 36300-37600 cm(-1) region. Thanks to the cooling of ions to ∼10 K, all the M(+)·B12C4 complexes show sharp vibronic bands in this region. For UV-UV hole-burning (HB) spectroscopy, we first check if our experimental system works well by observing UV-UV HB spectra of the K(+) complex with benzo-18-crown-6 (B18C6), K(+)·B18C6. In the UV-UV HB spectra of the K(+)·B18C6 complex, gain signals are also observed; these are due to vibrationally hot K(+)·B18C6 complex produced by the UV excitation of cold K(+)·B18C6 complex. Then we apply UV-UV HB spectroscopy to the M(+)·B12C4 complexes, and only one conformer is found for each complex except for the Li(+) complex, which has two conformers. The vibronic structure around the origin band of the UVPD spectra is quite similar for all the complexes, indicating close resemblance of the complex structure. The most stable structures calculated for the M(+)·B12C4 (M = Li, Na, K, Rb, and Cs) complexes also have a similar conformation among them, which coincides with the UVPD results. In these conformers the metal ions are too big to be included in the B12C4 cavity, even for the Li(+) ion. In solution, it was reported that 12-crown-4 (12C4) shows the preference of Na(+) ion among alkali metal ions. From the similarity of the structure for the M(+)·B12C4 complexes, it is suggested that the solvation of free metal ions, not of the M(+)·12C4 complexes, may lead to the selectivity of Na(+) ion for 12C4 in solution.

The H2O(+) radical ion, produced in an electrospray ion source via charge transfer from Eu(3+), is encapsulated in benzo-15-crown-5 (B15C5) or benzo-18-crown-6 (B18C6). We measure UV photodissociation (UVPD) spectra of the (H2O·B15C5)(+) and (H2O·B18C6)(+) complexes in a cold, 22-pole ion trap. These complexes show sharp vibronic bands in the 35 700-37 600 cm(-1) region, similar to the case of neutral B15C5 or B18C6. These results indicate that the positive charge in the complexes is localized on H2O, giving the forms H2O(+)·B15C5 and H2O(+)·B18C6, in spite of the fact that the ionization energy of B15C5 and B18C6 is lower than that of H2O. The formation of the H2O(+) complexes and the suppression of the H3O(+) production through the reaction of H2O(+) and H2O can be attributed to the encapsulation of hydrated Eu(3+) clusters by B15C5 and B18C6. On the contrary, the main fragment ions subsequent to the UV excitation of these complexes are B15C5(+) and B18C6(+) radical ions; the charge transfer occurs from H2O(+) to B15C5 and B18C6 after the UV excitation. The position of the band origin for the H2O(+)·B18C6 complex (36323 cm(-1)) is almost the same as that for Rb(+)·B18C6 (36315 cm(-1)); the strength of the intermolecular interaction of H2O(+) with B18C6 is similar to that of Rb(+). The spectral features of the H2O(+)·B15C5 complex also resemble those of the Rb(+)·B15C5 ion. We measure IR-UV spectra of these complexes in the CH and OH stretching region. Four conformers are found for the H2O(+)·B15C5 complex, but there is one dominant form for the H2O(+)·B18C6 ion. This study demonstrates the production of radical ions by charge transfer from multivalent metal ions, their encapsulation by host molecules, and separate detection of their conformers by cold UV spectroscopy in the gas phase.

The cooling of ionic species in the gas phase greatly simplifies the UV spectrum, which is of special importance when studying the electronic and geometric structures of large systems, such as biorelated molecules and host-guest complexes. Many efforts have been devoted to achieving ion cooling with a cold, quadrupole Paul ion trap (QIT), but one problem was the insufficient cooling of ions (up to ∼30 K) in the QIT. In this study, we construct a mass spectrometer for the ultraviolet photodissociation (UVPD) spectroscopy of gas-phase cold ions. The instrument consists of an electrospray ion source, a QIT cooled with a He cryostat, and a time-of-flight mass spectrometer. With great care given to the cooling condition, we can achieve ∼10 K for the vibrational temperature of ions in the QIT, which is estimated from UVPD spectra of the benzo-18-crown-6 (B18C6) complex with a potassium ion, K(+)·B18C6. Using this setup, we measure a UVPD spectrum of cold calix[4]arene (C4A) complex with potassium ion, K(+)·C4A. The spectrum shows a very weak band and a strong one at 36018 and 36156 cm(-1), respectively, accompanied by many sharp vibronic bands in the 36000-36600 cm(-1) region. In the geometry optimization of the K(+)·C4A complex, we obtain three stable isomers: one endo and two exo forms. On the basis of the total energy and UV spectral patterns predicted by density functional theory calculations, we attribute the structure of the K(+)·C4A complex to the endo isomer (C2 symmetry), in which the K(+) ion is located inside the cup of C4A. The vibronic bands of K(+)·C4A at 36 018 and 36 156 cm(-1) are assigned to the S1(A)-S0(A) and S2(B)-S0(A) transitions of the endo isomer, respectively.

The structures of estrogens (estrone(E1), β-estradiol(E2), and estriol(E3)) and their 1:1 hydrogen-bonded (hydrated) clusters with water formed in supersonic jets have been investigated by various laser spectroscopic methods and quantum chemical calculations. In the S1-S0 electronic spectra, all three species exhibit the band origin in the 35,050-35,200 cm(-1) region. By use of ultraviolet-ultraviolet hole-burning (UV-UV HB) spectroscopy, two conformers, four conformers, and eight conformers, arising from different orientation of OH group(s) in the A-ring and D-ring, are identified for estrone, β-estradiol, and estriol, respectively. The infrared-ultraviolet double-resonance (IR-UV DR) spectra in the OH stretching vibration are observed to discriminate different conformers of the D-ring OH for β-estradiol and estriol, and it is suggested that in estriol only the intramolecular hydrogen bonded conformer exists in the jet. For the 1:1 hydrated cluster of estrogens, the S1-S0 electronic transition energies are quite different depending on whether the water molecule is bound to A-ring OH or D-ring OH. It is found that the water molecule prefers to form an H-bond to the A-ring OH for estrone and β-estradiol due to the higher acidity of phenolic OH than that of the alcoholic OH. On the other hand, in estriol the water molecule prefers to be bound to the D-ring OH due to the formation of a stable ring-structure H-bonding network with two OH groups. Thus, the substitution of one hydroxyl group to the D-ring drastically changes the hydrogen-bonding preference of estrogens.

Because of the presence of polar groups in picolinamide, its aqueous solvation is characterized by formation of multiple hydrogen bonds, which are responsible for differences in vibrational spectra of picolinamide in the gas phase and its aqueous solution. In this contribution, we try to identify dominant interactions between the solute and the solvent molecules that are the origin of the observed spectral changes. For this purpose, we analyzed the vibrational properties of picolinamide (a single molecule and also embedded in the environment described with an implicit solvent model) and picolinamide-water dimers: by comparing their vibrational frequencies with experimentally observed values of picolinamides' aqueous solution and by analyzing the computed anharmonic force constants, it is possible to recognize whether and in which way the solvent causes changes in the vibrational properties of the title compound. Calculations performed on the picolinamide three-hydrate confirm conclusions obtained by analyzing monohydrates.

The structures of 17β-estradiol (estradiol) and its 1:1 cluster with water have been investigated in supersonic jets. The S(1)-S(0) electronic spectrum of estradiol monomer shows four strong sharp bands in the 35050-35200 cm(-1) region. Ultraviolet-ultraviolet hole-burning (UV-UV HB) and infrared-ultraviolet double-resonance (IR-UV DR) spectra of these bands indicate that they are due to four different conformers of estradiol originating from the different orientation of the OH groups in the A- and D-rings. The addition of water vapor to the sample gas generates four new bands in the 34700-34800 cm(-1) region, which are assigned to the estradiol-H(2)O 1:1 cluster with the A-ring (phenyl ring) OH acting as a hydrogen(H)-bond donor. In addition, we found very weak bands near the origin bands of bare estradiol upon the addition of water vapor. These bands are assigned to the isomers of estradiol-H(2)O 1:1 cluster having an H-bond at the D-ring OH. We determine the conformation of bare estradiol and the structures of its monohydrated clusters with the aid of density functional theory calculation and discuss the relationship between the stability of hydrated clusters and the conformation of estradiol.

A cluster anion of vinyl compounds in the gaseous phase has served as one of the simplest microscopic models of the initial stages of anionic polymerization. Herein, we describe our investigations into the initial stage mechanisms of anionic polymerization of acrylonitrile (AN; CH2═CHCN) trimer anions. While the cyclic oligomer is found in mass and photoelectron spectroscopic studies of (AN)3(-), only the chain oligomer is found in the infrared photodissociation (IRPD) spectrum of Ar-tagged (AN)3(-). On the basis of the calculated polymerization pathway of (AN)3(-), we consider that the chain oligomers are the reaction intermediates in the cyclization of (AN)3(-). The rotational isomerization of the (AN)3(-) chain oligomer is found to be the bottleneck in the cyclization of (AN)3(-). To form the (AN)4(-) chain oligomer by chain propagation, the addition of an AN molecule to (AN)3(-) should occur prior to the rotational isomerization. We conclude that the rotational isomerization in the (AN)3(-) chain oligomer is the key branching point between cyclization (termination) or chain propagation in the anionic polymerization.

Electronic and vibrational spectra of benzo-15-crown-5 (B15C5) and benzo-18-crown-6 (B18C6) complexes with alkali metal ions, M(+)•B15C5 and M(+)•B18C6 (M = Li, Na, K, Rb, and Cs), are measured using UV photodissociation (UVPD) and IR-UV double resonance spectroscopy in a cold, 22-pole ion trap. We determine the structure of conformers with the aid of density functional theory calculations. In the Na(+)•B15C5 and K(+)•B18C6 complexes, the crown ethers open the most and hold the metal ions at the center of the ether ring, demonstrating an optimum matching in size between the cavity of the crown ethers and the metal ions. For smaller ions, the crown ethers deform the ether ring to decrease the distance and increase the interaction between the metal ions and oxygen atoms; the metal ions are completely surrounded by the ether ring. In the case of larger ions, the metal ions are too large to enter the crown cavity and are positioned on it, leaving one of its sides open for further solvation. Thermochemistry data calculated on the basis of the stable conformers of the complexes suggest that the ion selectivity of crown ethers is controlled primarily by the enthalpy change for the complex formation in solution, which depends strongly on the complex structure.

The complexes between the host calix[4]arene (C4A) and various guest molecules such as NH(3), N(2), CH(4), and C(2)H(2) have been investigated via experimental and theoretical methods. The S(1)-S(0) electronic spectra of these guest-host complexes are observed by mass-selected resonant two-photon ionization (R2PI) and laser-induced fluorescence (LIF) spectroscopy. The IR spectra of the complexes formed in molecular beams are obtained by IR-UV double resonance (IR-UV DR) and IR photodissociation (IRPD) spectroscopy. The supramolecular structures of the complexes are investigated by electronic structure methods (density functional and second order perturbation theory). The current results for the various molecular guests are put in perspective with the previously reported ones for the C4A-rare gas (Rg) (Phys. Chem. Chem. Phys. 2007, 126, 141101) and C4A-H(2)O complexes (J. Phys. Chem. A, 2010, 114, 2967). The electronic spectra of the complexes of C4A with N(2), CH(4), and C(2)H(2) exhibit red-shifts of similar magnitudes with the ones observed for the C4A-Rg complexes, whereas the complexes of C4A with H(2)O and NH(3) show much larger red-shifts. Most of the IR-UV DR spectra of the complexes, except for C4A-C(2)H(2), show a broad hydrogen-bonded OH stretching band with a peak at ~3160 cm(-1). The analysis of the experimental results, in agreement with the ones resulting from the electronic structure calculations, suggest that C4A preferentially forms endo-complexes (guests inside the host calizarene cavity) with all the guest species reported in this study. We discuss the similarities and differences of the structures, binding energies, and the nature of the interaction between the C4A host and the various guest species.

The structure of the calix[4]arene(C4A)-(H(2)O) cluster formed in a supersonic beam has been investigated by mass-selected resonant two-photon ionization (R2PI) spectroscopy, IR-UV double resonance spectroscopy, IR photodissociation (IRPD) spectroscopy and by high-level quantum chemical calculations. The IR-UV double resonance spectrum of C4A-(H(2)O) exhibits a broad and strong hydrogen-bonded OH stretching band at 3160 cm(-1) and a weak asymmetric OH stretching band at 3700 cm(-1). The IRPD measurement of the cluster produced a value of 3140 cm(-1) for the C4A-(H(2)O) --> C4A + H(2)O dissociation energy. High-level electronic structure calculations at the MP2 level of theory with basis sets up to quadruple-zeta quality suggest that the endo-isomer (water inside the C4A cavity) is approximately 1100 cm(-1) more stable than the exo-isomer (water hydrogen bonded to the rim of C4A). The endo-isomer has a best-computed (at the MP2/aug-cc-pVQZ level) value of 3127 cm(-1) for the binding energy, just approximately 15 cm(-1) shy of the experimentally determined threshold and an IR spectrum in excellent agreement with the experimentally observed one. In contrast, the B3LYP density functional fails to even predict a stable structure for the endo-isomer demonstrating the inability of that level of theory to describe the delicate balance between structures exhibiting cumulative OH-pi H-bonding and dipole-dipole interactions (endo-isomer) when compared to the ones emanating from maximizing the cooperative effects associated with the formation of hydrogen bonded homodromic networks (exo-isomer). The comparison of the experimental results with the ones from high-level electronic structure calculations therefore unambiguously assign the endo-isomer as the global minimum of the C4A-(H(2)O) cluster, world's smallest cup of water.

Picosecond time-resolved IR-UV pump-probe spectroscopy has been performed to study intracluster vibrational energy redistribution (ICVR) and vibrational predissociation (VP) for the OH and CH stretch vibrations of phenol-ethylene hydrogen-bonded cluster. The transient UV spectra after the picosecond IR pulse excitation of these modes were observed by 1+1 REMPI via S(1) with a picosecond UV pulse. We have focused on the difference of the energy flow routes and their rates between the donor (phenol) and the acceptor (ethylene) site. Though the transient UV spectra showed a similar broad feature for all the vibrations examined, the time profiles exhibited a remarkable site dependence, as well as substantial mode dependence. Especially, we found a large difference in the early stage of the IVR evolution and the rates, whereas the VP rates were very similar.