Intramolecular Perturbations Research Articles

Astronomical CH3+ rovibrational assignments

Context. The methyl cation (CH3+) has recently been discovered in the interstellar medium through the detection of 7 μm (1400 cm−1) features toward the d203-506 protoplanetary disk by the JWST. Line-by-line spectroscopic assignments of these features, however, were unsuccessful due to complex intramolecular perturbations preventing a determination of the excitation and abundance of the species in that source. Aims. Comprehensive rovibrational assignments guided by theoretical and experimental laboratory techniques provide insight into the excitation mechanisms and chemistry of CH3+ in d203-506. Methods. The rovibrational structure of CH3+ was studied theoretically by a combination of coupled-cluster electronic structure theory and (quasi-)variational nuclear motion calculations. Two experimental techniques were used to confirm the rovibrational structure of CH3+:(1) infrared leak-out spectroscopy of the methyl cation, and (2) rotationally resolved photoelectron spectroscopy of the methyl radical (CH3). In (1), CH3+ ions, produced by the electron impact dissociative ionization of methane, were injected into a 22-pole ion trap where they were probed by the pulses of infrared radiation from the FELIX free electron laser. In (2), neutral CH3, produced by CH3NO2 pyrolysis in a molecular beam, was probed by pulsed-field ionization zero-kinetic-energy photoelectron spectroscopy. Results. The quantum chemical calculations performed in this study have enabled a comprehensive spectroscopic assignment of the v2+ and v4+ bands of CH3+ detected by the JWST. The resulting spectroscopic constants and derived Einstein A coefficients fully reproduce both the infrared and photoelectron spectra and permit the rotational temperature of CH3+ (T = 660 ± 80 K) in d203-506 to be derived. A beam-averaged column density of CH3+ in this protoplanetary disk is also estimated.

Fourier-transform spectroscopy, direct potential fit, and electronic structure calculations on the entirely perturbed (4)1Πstate of RbCs

We perform a high-resolution Fourier-transform spectroscopic study of the $(4){\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}$ state of the RbCs molecule by applying two-step $(4){\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}\ensuremath{\leftarrow}A{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+}\ensuremath{\sim}b{\phantom{\rule{0.16em}{0ex}}}^{3}\mathrm{\ensuremath{\Pi}}\ensuremath{\leftarrow}X{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+}$ optical excitation followed by observation of the $(4){\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}\ensuremath{\rightarrow}X{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+}$ laser-induced fluorescence (LIF) spectra. In many LIF progressions the collision-induced satellite rotational lines are observed, thus increasing the amount of term values and allowing us to estimate the $\mathrm{\ensuremath{\Lambda}}$-doubling effect in the $(4){\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}$ state. The direct potential fit (DPF) of experimental term values of 777 rovibronic levels of both $^{85}\mathrm{RbCs}$ and $^{87}\mathrm{RbCs}$ isotopologues is performed by means of the robust weighted nonlinear least-squares method. The DPF analysis based on the adiabatic approximation and analytical expanded Morse oscillator potential reveals numerous regular shifts in the measured level positions. The spectroscopic studies of the $(4){\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}$ state are supported by the electronic structure calculations including the potential energy curves of the singlet- and triplet-state manifold and spin-allowed transition dipole moments. The subsequent estimates of radiative lifetimes and corresponding vibronic branching ratios elucidate a dominant contribution of the $(4){\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}\ensuremath{\rightarrow}A\ensuremath{\sim}b$ channel into the total radiative decay of the $(4){\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}$ state. The relative intensity distributions simulated for $(4){\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}\ensuremath{\rightarrow}X{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+}$ LIF progressions agree well with their observed counterparts even for the profoundly shifted levels of the entirely perturbed $(4){\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}$ state. To get insight into the origin of the intramolecular perturbations, the relevant spin-orbit- and $L$-uncoupling electronic matrix elements are evaluated.

Collision-induced rovibrational energy transfer in small polyatomic molecules: the role of intramolecular perturbations

ABSTRACTVarious forms of time-resolved optical double-resonance spectroscopy facilitate rotationally resolved measurements of collision-induced intramolecular vibration-to-vibration (V–V) energy-transfer processes, which take a gas-phase polyatomic molecule from one distinct rovibrational energy level to another. Of longstanding mechanistic interest are questions concerning the extent to which such V–V energy transfer (ET) may be influenced by intramolecular perturbations – notably Fermi resonance (and other anharmonic mixing effects) and Coriolis coupling – within polyatomic molecular rovibrational manifolds of interest. It is evident that quantum-mechanical interference effects can arise, either inhibiting or enhancing the probability of collision-induced ET in perturbed rovibrational manifolds of certain small gas-phase polyatomic molecules, notably CO2, D2CO and C2H2. This article focuses on a blend of high-resolution rovibrational spectroscopy (characterising initial and final molecular levels and their intramolecular perturbations) and collision dynamics (with colliding molecules defined in terms of isolated-molecule spectroscopic basis states). It aims to offer fresh insights and to consider some apparent mechanistic anomalies (e.g. collision-induced quasi-continuous background effects in the 4νCH rovibrational manifold of C2H2). Various reported experiments and related theoretical treatments are critically re-examined, in order to pose and address mechanistic questions some of which still challenge detailed understanding.

Invariance of molecular response properties under a coordinate translation

The response of a molecule to static and dynamic electromagnetic fields and intramolecular perturbations is reviewed within the framework of the Rayleigh–Schrödinger perturbation theory. A semiclassical approach is adopted, using quantized form of electronic operators and the nonquantized description of the electromagnetic fields via the Maxwell equations. The Bloch multipolar gauge has been used to define operators suitable to describe the molecular interaction with nonhomogneous time‐dependent perturbations. It is shown that the quantum mechanical theory of magnetic properties can profitably be proposed in terms of electronic current densities induced by an external magnetic field and permanent magnetic dipole moments at the nuclei. Theoretical relationships are reported to evaluate magnetizability, nuclear magnetic shielding, and nuclear spin‐spin coupling, proving that the whole theory can be reformulated via the equations of classical electromagnetism, provided that the current density is evaluated by quantum mechanical methods. Emphasis is placed on the invariance of response properties in a translation of the coordinate system as a basic requirement for measurability. The connections among translational invariance, gauge invariance, and electron charge conservation are outlined, showing that they can be illustrated via quantum mechanical sum rules. A number of relationships describing the change of static and dynamic electromagnetic properties in a translation of the reference frame are reported. © 2014 Wiley Periodicals, Inc.

Spectroscopy and energetics of the acetylene molecule: dynamical complexity alongside structural simplicity

This article reviews laser-spectroscopic studies of the structure, energetics, and dynamics of processes involving small polyatomic molecules, particularly acetylene (ethyne, C2H2). The linear, centrosymmetric structure of C2H2 is deceptively simple, given that aspects of its optical spectra and dynamics have proved to be unusually complicated. The article focuses on the ground electronic state of C2H2, where rovibrational eigenstates are only approximately described in normal-mode terms, because intramolecular processes (such as anharmonic mixing, ℓ-type resonances, and Coriolis coupling) introduce extensive global and local perturbations. These tend to spoil quantum numbers and symmetries that are well-defined in low-order basis states. Such effects within the rovibrational energy states of C2H2 are systematically characterized, together with dynamical descriptions in terms of polyad models and insight into photochemical or photophysical processes that may occur at high vibrational energies, without direct electronic excitation. Time-resolved optical double-resonance spectroscopy, probed by ultraviolet-laser-induced fluorescence and pumped by either infrared absorption or coherent Raman excitation, has proved particularly useful in exploring such effects in gas-phase C2H2; techniques of this type are discussed in detail, together with other laser-spectroscopic methods that provide complementary mechanistic information. A closely related topic concerns the area of optothermal molecular-beam spectroscopy, with particular emphasis on research by the late Roger E. Miller to whose memory this article is dedicated. Key publications by Miller and coworkers, in many of which C2H2 and its isotopomers play a central role, are reviewed. These cover the following themes: structure of molecular complexes and clusters, infrared predissociation spectra, rotational and vibrational energy transfer, differential scattering, photofragmentation of oriented complexes, superfluid-helium nanodroplet spectroscopy, aerosols formed in low-temperature diffusion cells, surface scattering experiments, optically selected mass spectrometry, and characterization of biomolecules. A unifying issue that links the assorted topics of this article is the role that intramolecular perturbations can play to enhance (and sometimes suppress) the efficiency of rovibrational energy transfer in colliding molecules or in molecular complexes and clusters; C2H2 and its isotopomers have been a rich source of insight in this regard, although they continue to pose challenges to our understanding. Contents PAGE 1. Introduction 656 2. The occurrence of acetylene: connections and coincidences 657 3. Spectroscopic complexities of acetylene 659 3.1. Vibrational levels in the [Xtilde] electronic ground state 659 3.2. Anharmonic perturbations in the 4νCH vibrational manifold of C2H2 663 3.3. Local J-dependent anharmonic and ℓ-resonance perturbations 665 3.4. Local J-dependent heterogeneous Coriolis-coupling perturbations 668 3.5. Local Stark field perturbations in electric fields 671 3.6. Electronically excited states of C2H2 674 4. Acetylene as a mechanistic probe: Part of the Miller legacy 676 4.1. Optothermal spectroscopy of complexes and clusters containing C2H2 677 4.2. PHOFAD: Photofragmentation of oriented C2H2-containing complexes 680 4.3. Liquid helium nanodroplets incorporating C2H2 and its complexes 684 5. Time-resolved optical double-resonance spectroscopy of acetylene 688 5.1. Raman-ultraviolet double-resonance spectroscopy of acetylene 689 5.2. Infrared-ultraviolet double-resonance spectroscopy of acetylene 692 5.2.1. IR-UV DR studies of low-energy bending levels of acetylene 693 5.2.2. IR-UV DR studies in the νCH manifold of C2H2 695 5.2.3. IR-UV DR studies in nνCH manifolds of C2H2 above ∼6500 cm−1 698 5.2.4. IR-UV DR studies in the (νCC + 3νCH) manifold of C2H2 at ∼11 600 cm−1 699 5.2.5. IR-UV DR studies in the 4νCH manifold of C2H2 at ∼12 700 cm−1 702 5.2.6. IR-UV DR rovibrational spectroscopy of the C2H2-Ar van der Waals complex 706 5.3. Techniques that complement IR-UV DR spectroscopy of acetylene 707 6. Concluding remarks 709 Acknowledgments 710 References 710

Rovibrational Energy Transfer in the 4νCH Manifold of Acetylene, Viewed by IR-UV Double Resonance Spectroscopy. 3. State-to-State J-Resolved Kinetics

Abstract Time-resolved infrared-ultraviolet double resonance (IR-UV DR) spectroscopy is used to study the kinetics of collision-induced state-to-state molecular energy transfer between rovibrational states in the 12700-cm−1 4νCH manifold of the electronic ground state of acetylene (C2H2). Particular initial and final rovibrational J-states are prepared and monitored by a pair of tunable laser pulses (IR PUMP and UV PROBE) and the kinetic results recorded by continuously varying the time delay between those pulses at a set sample pressure. After allowing for collision-induced quenching of fluorescence and mass transfer from the IR-UV optical excitation zone (by beam flyout and diffusion), an array of kinetic data for J-resolved energy-transfer channels can be interpreted by means of a mechanistically structured master-equation model. This paper focuses on kinetics derived by probing C2H2 in its 4νCH J = 12 state (which is affected by intramolecular perturbations and implicated in unusual collision-induced quasi-continuous background effects) and J-resolved collision-induced rovibrational energy transfer with both even ΔJ and (supposedly forbidden) odd ΔJ.

Rovibrational Energy Transfer in the 4νCHManifold of Acetylene Viewed by IR−UV Double Resonance Spectroscopy. 2. Perturbed States withJ= 17 and 18

Collision-induced state-to-state molecular energy transfer between rovibrational states in the 12,700 cm(-1) 4nu(CH) manifold of the electronic ground state X of acetylene (C(2)H(2)) is monitored by time-resolved infrared-ultraviolet double resonance (IR-UV DR) spectroscopy. Rotational J-states associated with the (nu(1) + 3nu(3)) or (1 0 3 0 0)(0) vibrational combination level, initially prepared by an IR pulse, are probed at approximately 299, approximately 296, or approximately 323 nm with UV laser-induced fluorescence via the Alpha electronic state. The rovibrational J-states of interest belong to a congested manifold that is affected by anharmonic, l-resonance, and Coriolis couplings, yielding complex intramolecular dynamics. Consequently, collision-induced rovibrational satellites observed by IR-UV DR comprise not only regular even-DeltaJ features but also supposedly forbidden odd-DeltaJ features. A preceding paper (J. Phys. Chem. A 2003, 107, 10759) focused on low-J-value rovibrational levels of the 4nu(CH) manifold (particularly those with J = 0 and J = 1) whereas this paper examines locally perturbed states at higher values of J (particularly J = 17 and 18, which display anomalous doublet structure in IR-absorption spectra). Three complementary forms of IR-UV DR experiments (IR-scanned, UV-scanned, and kinetic) are used to address the extent to which intramolecular perturbations influence the efficiency of J-resolved collision-induced energy transfer with both even and odd DeltaJ.

The substituent effect in ethylenes and acetylenes – AIM analysis

The calculations on disubstituted ethylenes YCH CHX with Y = Li, H, F and X = H, F, Li, Na, OH, BeH, NH 2, BH 2, NO 2 were performed at MP2/6-311++G(d,p) level of theory. The analysis of bond lengths and atoms in molecules based theory (AIM) topological parameters such as the characteristics of bond critical points (electron densities and their Laplacians) and atomic radii leads to the conclusion that the AIM parameters are much more sensitive to the action of intramolecular perturbations like substituents than traditional structural parameters such as bond lengths. A comparison of substituted ethylenes with the previously analyzed substituted acetylenes is also given.

Where the two carbon atoms touch in the triple bond in disubstituted acetylenes: the AIM analysis

Optimization of 24 disubstituted acetylenes YCCX with Y=Li, H, F and X=H, F, Li, Na, OH, BeH, NH 2, BH 2, NO 2 was carried out at the MP2/6-311++G(d,p) level of theory. The analysis of structural parameters (bond lengths, atomic radii) and atoms in molecules theory (AIM) topological parameters (electron densities and their Laplacians in bond critical points) leads to the conclusion that the AIM parameters are much more sensitive to the action of intramolecular perturbations (like substituents) than traditional structural parameters like bond lengths (or bond angles). In particular, the shape of the atom strongly depends on electronegativity of its neighboring atom in the molecule and the effect is transmitted even over a few bonds.

A fast ab initio model for the calculation of excited electronic states of atoms and molecules in a weakly polarizable environment. I. Theory

We report the development of an ab initio scheme designed for the calculation of the electronic ground state and low-lying excited states of an atom or a molecule, perturbed by a weakly interacting environment of discrete, unpolarizable particles acting as a solvent. The model employs an ab initio partitioning ansatz that accounts for the effects of nonlocal exchange–overlap interactions between the solute and the solvent by means of a parametrized exchange–overlap operator and an effective metric in the pair-permutation, pair-additivity approximation, which is known to be valid in regions of small intermolecular overlap. Intramolecular perturbations like spin-orbit effects can be incorporated in the treatment. Due to its fast performance and built-in size-consistency, our model can be employed in the calculation of the electronic states of spectroscopically active fragments with many different settings of the environment.

The evolution and revival structure of angular momentum quantum wave packets

In this paper, a coherent superposition of angular-momentum states created by absorption of polarized light by molecules is analyzed. Attention is paid to the time evolution of wave packets representing the spatial orientation of the internuclear axis of a diatomic molecule. Two examples are considered in detail. Molecules absorbing light in a permanent magnetic field experiencing the Zeeman effect and molecules absorbing light in a permanent electric field experiencing the quadratic Stark effect. In a magnetic field, we have a wave packet that evolves in time exactly as a classical dipole oscillator in a permanent magnetic field (classical-physics picture of the Zeeman effect). In the second case, we have a wave packet that goes through periodical changes of shape of the packet with revivals of the initial shape. This is pure quantum behavior. The classical motion of angular momentum in an electric field in the case of a quadratic Stark effect is known to be a periodic. Solutions obtained for wave packet evolution are briefly compared with Rydberg-state coherent wave packets and harmonic-oscillator wave packets. Zeeman and Stark effects in small molecules continuously attract the attention of researchers, theoreticians, as well as experimentalists. These investigations allow us to obtain a deeper understanding of the interaction of molecules with stationary external fields and also can be used as a practical tool to measure different molecular characteristics, such as permanent electric or magnetic dipole moments, intramolecular perturbations, etc. It is worthwhile analyzing these effects as an evolution of wave packets. All this motivates a comparison of the quantum and classical picture of Zeeman and Stark effects in molecules.PACS No.: 33.55.Be

The evolution and revival structure of angular momentum quantum wave packets

In this paper, a coherent superposition of angular-momentum states created by absorption of polarized light by molecules is analyzed. Attention is paid to the time evolution of wave packets representing the spatial orientation of the internuclear axis of a diatomic molecule. Two examples are considered in detail. Molecules absorbing light in a permanent magnetic field experiencing the Zeeman effect and molecules absorbing light in a permanent electric field experiencing the quadratic Stark effect. In a magnetic field, we have a wave packet that evolves in time exactly as a classical dipole oscillator in a permanent magnetic field (classical-physics picture of the Zeeman effect). In the second case, we have a wave packet that goes through periodical changes of shape of the packet with revivals of the initial shape. This is pure quantum behavior. The classical motion of angular momentum in an electric field in the case of a quadratic Stark effect is known to be a periodic. Solutions obtained for wave packet evolution are briefly compared with Rydberg-state coherent wave packets and harmonic-oscillator wave packets. Zeeman and Stark effects in small molecules continuously attract the attention of researchers, theoreticians, as well as experimentalists. These investigations allow us to obtain a deeper understanding of the interaction of molecules with stationary external fields and also can be used as a practical tool to measure different molecular characteristics, such as permanent electric or magnetic dipole moments, intramolecular perturbations, etc. It is worthwhile analyzing these effects as an evolution of wave packets. All this motivates a comparison of the quantum and classical picture of Zeeman and Stark effects in molecules.PACS No.: 33.55.Be

Coupling of a Jahn–Teller pseudorotation with a hindered internal rotation in an isolated molecule: 9-hydroxytriptycene

The irregular vibronic structure in the S1←S0 resonant two-photon ionization (R2PI) spectrum of supersonically cooled triptycene is a result of a classic E⊗e Jahn–Teller effect [A. Furlan et al., J. Chem. Phys. 96, 7306 (1992)]. This is well characterized and can be used as an effective probe of intramolecular perturbations. Here we examine the S1←S0 R2PI spectrum of 9-hydroxytriptycene and the fluorescence from various excited state vibronic levels. In this system the pseudorotation of the Jahn–Teller vibration is strongly coupled to the torsional motion of the bridgehead hydroxy group. This torsional motion results in a tunneling splitting in both the ground and excited states. The population of the upper level in the ground electronic state results in additional vibronic transitions becoming symmetry allowed in the R2PI spectrum that are forbidden in the bare triptycene molecule. The assignment of the R2PI and fluorescence spectra allows the potential energy surfaces of these vibrational modes to be accurately quantified. The full C3v vibronic point group must be used to interpret the spectra. The time scale of the internal rotation of the–OH group and the butterfly flapping of the Jahn–Teller pseudorotation are of similar magnitude. The tunneling between the nine minima on the three dimensional potential energy surface is such that the Jahn–Teller pseudorotation occurs in concert with the–OH internal rotation. The Berry phase that is acquired during this motion is discussed. The simple physical picture emerges of the angle between two of the three benzene moieties opening in three equivalent ways in the S1 electronic state. This geometry follows the position of the hydroxy group, which preferentially orients itself to point between these two rings.

Proteolytic refolding of the HIV-1 capsid protein amino-terminus facilitates viral core assembly.

After budding, the human immunodeficiency virus (HIV) must 'mature' into an infectious viral particle. Viral maturation requires proteolytic processing of the Gag polyprotein at the matrix-capsid junction, which liberates the capsid (CA) domain to condense from the spherical protein coat of the immature virus into the conical core of the mature virus. We propose that upon proteolysis, the amino-terminal end of the capsid refolds into a beta-hairpin/helix structure that is stabilized by formation of a salt bridge between the processed amino-terminus (Pro1) and a highly conserved aspartate residue (Asp51). The refolded amino-terminus then creates a new CA-CA interface that is essential for assembling the condensed conical core. Consistent with this model, we found that recombinant capsid proteins with as few as four matrix residues fused to their amino-termini formed spheres in vitro, but that removing these residues refolded the capsid amino-terminus and redirected protein assembly from spheres to cylinders. Moreover, point mutations throughout the putative CA-CA interface blocked capsid assembly in vitro, core assembly in vivo and viral infectivity. Disruption of the conserved amino-terminal capsid salt bridge also abolished the infectivity of Moloney murine leukemia viral particles, suggesting that lenti- and oncoviruses mature via analogous pathways.

Molecular Mechanisms for the Optical Activities of Polyisocyanates Induced by Intramolecular Chiral Perturbations

Molecular mechanisms were proposed to account for extraordinary optical activities of polyisocyanates containing chiral moieties (chiral monomer or initiator fragment) reported by Green et al. and by Okamoto et al., where the polyisocyanate molecule was modeled by a helical chain consisting of an alternating sequence of right-handed and left-handed helices with helix reversals in between. Theoretical predictions based on this model were consistent with remarkable dependence of optical rotation on monomer composition for random copolyisocyanates of chiral and achiral monomers and enantiomorphous copolymers and chain length dependence of optical rotation for achiral polyisocyanates with a chiral initiator fragment on the chain end. The helix reversal was shown to be the key factor for all the cases and more frequent for aromatic sidechain polymer poly(m-methyl phenyl isocyanate) than for aliphatic side chain polymers such as poly(butyl and hexyl isocyanate)s.

Magnetic field induced alignment–orientation conversion: Nonlinear energy shift and predissociation in Te2 B1u state

The paper analyzes magnetic field induced alignment–orientation conversion (AOC) phenomenon caused by simultaneous effect of quadratic terms in Zeeman energy shift and magnetic predissociation (PD), producing asymmetry either in energy splitting ωMM±1≠ω−M∓1−M or in relaxation of coherence ΓMM±1≠Γ−M∓1−M between coherently excited M, M±1 magnetic sublevels. The AOC is registered via the appearance of circular polarization (C) of fluorescence under linearly polarized excitation. The unified perturbation treatment of a molecule in external magnetic field B is presented, accounting for magnetic and intramolecular perturbations via interaction with bonded or continuum states, considering Hund’s (c)-case coupling and dividing the intramolecular perturbation operator into homogeneous (ΔΩ=0) and heterogeneous (ΔΩ=±1) parts. Explicit expressions up to B2 terms are given for energy shift and PD rate, adapted to 1u state in conditions relevant to the B 3Σ−u complex of Te2 molecule. Numeric simulation revealed that nonlinear magnetic energy shift and heterogeneous magnetic PD produce dispersion type fluorescence circularity signals C(B) of different sign. Fitting of experimental data on B1−u, v(J)=2(96) state of 130Te2 molecule allowed to determine the electronic matrix element of paramagnetic Hamiltonian (Ω=0|Ĥpm|Ω=1)≡G±=2.7, as well as the natural Cvhet=±6 s−1/2 and the magnetic αvhet=∓9×103 s−1/2 T−1 rate constants of heterogeneous PD, supposing that the B1−u state PD takes place through 0−u state continuum. As a result, magnetic AOC represents a sensitive method to investigate molecular structure and intramolecular interaction between both bonded and continuum states. Additionally, it has been shown that the magnetic PD effect leads to strong amplification of nonzero field level crossing signals caused by B2 terms in Zeeman energy shift.

Collision-induced state-to-state energy transfer in perturbed rovibrational manifolds of small polyatomic molecules: mechanistic insights and observations

Abstract This paper considers collision-induced rovibrational energy transfer in small polyatomic molecules, measured on a state-to-state basis by rotationally selective, time-resolved optical double-resonance (DR) spectroscopy. Particular attention is given to the extent to which the rovibrational manifold of interest is energetically congested and/or affected by intramolecular perturbations. A semiclassical dynamical description is used to visualise how such perturbations, manifested in spectroscopic energies and transition probabilities, affect the efficiencies of collision-induced bovibrational energy transfer. Perturbation-induced enhancement (and, in some cases, inhibition) of V-V transfer efficiencies is examined, with regard to several DR measurements of perturbed manifolds of CO 2 , D 2 CO, C 2 H 2 , and C 2 D 2 at various levels of vibrational excitation. Specific questions considered include: the feasibility of separating rotationally specific channels of collision-induced V-V transfer from the scrambling effects of rotational relaxation; how to characterise dynamical processes in which intermolecular perturbations exceed (and effectively override) those of intramolecular origin; the role of vibrational angular momentum in propensities and efficiencies of collision-induced V-V transfer; physically realistic molecular bases to minimise perturbation-induced contributions to rovibrational energy transfer; reconciliation of apparently contrasting dynamics of small and large polyatomic molecules, over a range of vibrational energies, spectral congestion, and molecular complexity.

The weakening of the intramolecular hydrogen bond NH⋯O in complexes of ortho-substituted anilines with proton acceptors

The IR spectra of ortho-nitroaniline and methyl anthranilate and their complexes with protonacceptors B have been studied in the region of the stretching vibrations of the NH 2, ND 2 and NHD groups. Weakening of the intramolecular hydrogen bond NH⋯O under the influence of the intermolecular bond NH⋯B has been found to occur. The cooperativity effect increases with the strength of the inter- and intramolecular hydrogen bonds. A method of estimating the intramolecular hydrogen-bond enthalpy in ortho-substituted anilines has been proposed, based on the assumption of the equality of two hydrogen-bond enthalpies in the complex with the symmetrically perturbed amino group. The equivalence of inter- and intramolecular perturbations were determined by the coincidence of cis- and trans-NH (and/or ND) frequencies of the NHD group.

Intramolecular Perturbations in Small Molecules in the Presence of a Continuous Electric Field

Intramolecular Perturbations in Small Molecules in the Presence of a Continuous Electric Field

Intramolecular perturbations and collisional energy transfer: Classical and quantum theory

The effect of resonant intramolecular interactions upon vibrational energy transfer is studied using classical and quantum perturbation theory. It is shown that cancellations attributed previously to quantum interference may equally well be ascribed to a classical mechanism.