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Mass-selective two-color resonant two-photon ionization (2C-R2PI), UV/UV hole-burning, and infrared (IR) depletion spectra of supersonic jet-cooled 2-pyridone·(H2O)nclusters with n= 1–4 have been measured to investigate the local hydration patterns around 2-pyridone (2PY) as a function of cluster size. As shown by others, the IR frequencies of the OH and NH stretches of the n= 1, 2 clusters are characteristic of water wires stretching from the NH to the C═O group of 2PY. We identify two isomers (3A and 3B) of the n= 3 cluster in the 2C-R2PI spectrum and separate them by IR/UV and UV/UV hole-burning techniques. Isomer 3A exhibits a three-membered water wire, extending the n= 1, 2 structural motif. Isomer 3B exhibits bifurcated water wires with the first H2O donating to two waters that form H-bonds to the C═O group. This increases the H-bond strength between the NH group of 2PY and the proximal H2O molecule, lowering the NH stretch to ∼2800 cm–1. The n= 4 cluster is also bifurcated with two water wires between the bifurcating H2O and the C═O group. The cluster-selective IR spectra are complemented with density-functional calculations using the PW91, B3LYP, B97-D, and M06-2X functionals, where the latter two include long-range dispersive interactions, and with the ab initio correlated SCS-CC2 method. The calculated IR spectra provide firm assignments of the structures of the n= 1–4 cluster structures and allow us to understand the evolution of individual H-bond strengths with increasing cluster size.

Narrowing the gap: A compactly fused π‐conjugated molecule combines a high‐lying HOMO with a low‐lying LUMO (Eox−Ered=0.52 eV) and a fairly low‐lying LUMO+1 on the bridging unit, giving rise to strong optical charge‐transfer transitions. A facile electron transfer has been observed by EPR and NMR spectroscopies.

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To study the electronic interactions in donor–acceptor (D–A) ensembles, D and A fragments are coupled in a single molecule. Specifically, a tetrathiafulvalene (TTF)‐fused dipyrido[3,2‐a:2′,3′‐c]phenazine (dppz) compound having inherent redox centers has been synthesized and structurally characterized. Its electronic absorption, fluorescence emission, photoinduced intramolecular charge transfer, and electrochemical behavior have been investigated. The observed electronic properties are explained on the basis of density functional theory.

We review experimental and theoretical investigations of excited-state hydrogen atom transfer (ESHAT) reactions along unidirectionally hydrogen bonded solvent 'wire' clusters. The solvent wire is attached to the aromatic 'scaffold' molecule 7-hydroxyquinoline (7HQ), which offers an O-H and an N hydrogen bonding site, spaced far enough apart to form two- to four-membered wires. S1 ← S0 photoexcitation renders the O-H group more acidic and the quinolinic N more basic. This provides a driving force for the enol → keto tautomerization, probed by the characteristic fluorescence of the 7-ketoquinoline in the molecular beam experiments. For 7-hydroxyquinoline·(NH3)3, excitation of ammonia-wire vibrations induces the tautomerization at ∼200 cm-1. Different reaction pathways have been explored by excited-state ab initio calculations. These show that the reaction proceeds by H-atom transfer along the wire as a series of Grotthus-type translocation steps. There is no competition with a mechanism involving successive proton translocations. The rate-controlling S1 state barriers arise from crossings of a π π* with a Rydberg-type πσ* state and the proton and electron movements along the wire are closely coupled. The excited state reactant, H-transferred intermediates and product structures are characterized. The reaction proceeds by tunnelling, as shown by deuteration of the solvent molecules (ND3) in the wire. The first step of the reaction exhibits intra/intermolecular vibrational mode selectivity. Substitution of NH3 by one, two or three H2O molecules in the wire leads to increasing threshold with each additional H2O molecule, up to >2000 cm-1 for the 7-hydroxyquinoline·(H2O)3 water-wire cluster. No 7-ketoquinoline fluorescence is observed upon insertion of even a single H2O molecule. The calculations show that insertion of each H2O molecule into the solvent wire introduces a high barrier, which blocks any further H-atom transfer.Contents PAGE1. Introduction 4582. 7-Hydroxyquinoline 4602.1. Ground and excited state acid/base properties 4602.2. O-H bond breaking in the ππ* and πσ* states 4613. The 7-hydroxyquinoline · (NH3)n ammonia-wire clusters: an overview 4633.1. Structures and electronic origins 4633.2. Resonant two-photon ionization spectra 4664. Excited-state hydrogen atom transfer in 7-hydroxyquinoline · (NH3)3 4684.1. UV-UV depletion and fluorescence action spectra 4684.2. Two reaction paths: ESHAT vs. ESPT 4704.3. The enol → HT1 reaction coordinate 4744.4. Vibrational mode selectivity 4745. Solvent e.ects on excited state H-atom transfer: mixed ammonia/water clusters 4795.1. Comparison of 7HQ· (H2O)3 and 7HQ · (NH3)3 4795.2. The mixed 7HQ · (NH3)2 · H2O and 7HQ · NH3 · (H2O)2 clusters 4825.3. Discussion 4846. Conclusion 485References 487

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The energetics and infrared IR spectrum of the double proton transfer DPT in the hydrogen bonded 2-pyridone·2-hydroxypyridine 2PY·2HP dimer are studied using quantum chemical and classical molecular dynamics methods and compared with recent experimental work. The experimental spectrum shows IR absorption between 2500 and 3000 cm−1with peak absorption at 2700 cm−1. High resolution spectroscopy of the S1 ← S0origin of 2PY·2HP have revealed a tunneling splitting which was attributed to DPT. MP2 and density functional calculations predict DPT barriers between 8 and 8.5 kcal mol−1in the electronic ground state. Two-dimensional potential energy surfaces and the minimum-energy reaction path MEP for double proton transfer are calculated: Near the minima, the MEP is dominated by the 2PY·2HP intermolecular stretch, whereas close to the transition state the MEP corresponds to almost purely biprotonic exchange motion. Classical MD trajectories are calculated using the self consistent charges–density functional tight binding method at six different internal energies. Using activated dynamics, classical IR spectra are calculated from the time-dependence of the dipole moment function. The first 500 fs of the dynamics show broad spectral features in the mid-IR between 2000–2800 cm−1, as found in the experiment. These early phases of the activated dynamics are associated with large-amplitude vibrations of the N–H and O–H protons and may lead to double proton transfer. After 500 fs, the broad IR features disappear from the 2000–2800 cm−1range.

The energetics and infrared (IR) spectrum of the double proton transfer (DPT) in the hydrogen bonded 2-pyridone·2-hydroxypyridine (2PY·2HP) dimer are studied using quantum chemical and classical molecular dynamics methods and compared with recent experimental work. The experimental spectrum shows IR absorption between 2500 and 3000 cm−1 (with peak absorption at 2700 cm−1). High resolution spectroscopy of the S₁ ← S₀ origin of 2PY·2HP have revealed a tunneling splitting which was attributed to DPT. MP2 and density functional calculations predict DPT barriers between 8 and 8.5 kcal mol−1 in the electronic ground state. Two-dimensional potential energy surfaces and the minimum-energy reaction path (MEP) for double proton transfer are calculated: Near the minima, the MEP is dominated by the 2PY·2HP intermolecular stretch, whereas close to the transition state the MEP corresponds to almost purely biprotonic exchange motion. Classical MD trajectories are calculated using the self consistent charges–density functional tight binding method at six different internal energies. Using activated dynamics, classical IR spectra are calculated from the time-dependence of the dipole moment function. The first ≈500 fs of the dynamics show broad spectral features in the mid-IR between 2000–2800 cm−1, as found in the experiment. These early phases of the activated dynamics are associated with large-amplitude vibrations of the N–H and O–H protons and may lead to double proton transfer. After 500 fs, the broad IR features disappear from the 2000–2800 cm−1 range.

Time-resolved degenerate four-wave mixing experiments in the femtosecond time domain were employed to record the rotational ground-state dynamics of the N-heteroaromatic molecules pyridine, pyrimidine and pyrazine in the gas phase. Pyrimidine is an important molecular building block of several nucleic acid bases. The method records the time-dependent change in third-order polarization of a sample after coherent excitation. All three molecules are asymmetric tops in the oblate limit. From the analysis of the transients we obtained highly accurate averages of the two larger ground-state rotational constants A and B. Copyright © 2002 John Wiley & Sons, Ltd.

The optical absorption spectra of the à 2Π3/2,g→X˜2Π3/2, u transitions of dichloro-, dibromo- and diiodoacetylene cations in 4·5 K neon matrices have been obtained. The spectra are vibrationally analysed and yield the frequencies of three or four of the fundamental vibrations of the cations in their Ã2Π3/2,g electronic states.

Emission spectra of 1,3,5-C6F3H3+(h3+) and 1,3,5-C6F3D3+(d3+) ions were obtained by electron impact in seeded He and Ar supersonic free jets. The h3+ (and d3+) spectra were recorded between 4570 (4445) Å and 4820 (4810) Å with a resolution R of the order of 0·5 cm-1.Gas phase discharge emission spectra of the same ions were photographed at R=0·25 cm-1 from 4450 to 4800 Å and R=0·1 cm-1, between 4500 and 4700 Å, and are compared to the jet cooled spectra. This comparison leads to some new or refined spectral assignments. Jet cooled and Ne matrix ion emission spectra are also compared, particularly with regard to frequencies and relative intensities of bands exhibiting the Jahn-Teller effect in the B˜2A''2-X˜2E'' transition. Matrix dependent effects on Jahn-Teller levels and interaction parameters are observed and discussed.Rotational contour analysis of peak structures in the 00/0 and 61,1/2/0,0 emission bands in the discharge and jet spectra of the h3+ ion indicates that ΔB ⋍ -0·0020 ± 0·0005 cm-1 and that the ground state Coriolis coefficients are ζ''0,1/2 ⋍ -0·8 to -0·6 and ζ''1,1/2 ⋍ -0·2 to +0·2. Possible consequences of spin-orbit interaction effects on the band contours are considered.

The photoionization efficiency curves for 39,39K2and 39,41K2dimers excited to the B 1Πuelectronic state have been measured using sequential two-photon-ionization techniques. Accurate determinations of nine photoionization thresholds yield an adiabatic ionization energy of 4.06073 ± 0.00016 eV. Autoionizing Rydberg states are assigned and analyzed. Autoionization rates are measured for several Rydberg vibronic states. The isotopic dependence of the autoionization structure is partially analyzed and molecular constants for the K+2(2Σ+g) state are derived.

Intermolecular bonding between a large polar aromatic molecule (carbazole) and a rare gas atom (Kr) is characterized by several laser spectroscopic methods. Intermolecular vibrations and bond dissociation energies are measured for the Soand S1states. Detailed tests of model calculations incorporating dispersive and inductive interactions indicate that the latter contribution to the binding energy is about 15%.

A planar π-conjugated heteroaromatic molecule 1has been synthesized and fully characterized; it combines two characteristics, a charge-transfer transition originating from its inherent donor–acceptor nature in its neutral state and an intervalence charge-transfer transition in its 12+mixed-valence state.

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