Overtone States Research Articles

Anharmonic coupling behind vibrational spectra of solvated ammonium: lighting up overtone states by Fermi resonance through tuning solvation environments

Studies on the vibrational spectra of various ammonium-centered clusters under different solvation environments have raised interest over the last thirty years. The gas-phase infrared photodissociation spectroscopy (IRPD) experiments showed that these NH4+Xn clusters exhibit rich spectral features from 2600 to 3400 cm-1. In this work, we have simulated the vibrational spectra and analyzed couplings among vibrational quantum states in the aforementioned frequency range using ab initio anharmonic algorithms. Originating from the anharmonic couplings between NH stretching fundamentals and bending overtones, Fermi resonance (FR) is a common feature in these spectra, and its extent is determined by the magnitude of couplings and the energy matching conditions between relevant states, which are governed by the proton affinity, number, and bonding configuration of the solvation species. For weakly bound clusters consisting of rare gas atoms, FR is insignificant but not negligible; for strongly bound clusters, such as ammonium-water clusters, the hydrogen-bonded NH stretching fundamentals redshift and reach a better resonance condition, and thus light up the bending overtones as prominent FR bands. Our simulated spectra are in good agreement with previous experimental reports of these ammonium-centered clusters and provide a better understanding of the vibrational coupling behind the spectra of the NH stretching region.

Millimeter-wave spectroscopy of the chlorine isotopologues of 2-chloropyridine and twenty-three of their vibrationally excited states

A combined total of 25 vibrational states of 2-chloropyridine (C5H4NCl, μa = 3.07 D, μb = 1.70 D), including states for both chlorine isotopologues, have been least-squares fit to sextic, A-reduced Hamiltonians with low error (<0.05 MHz). In total, over 22,500 transition frequencies were measured in the 135–375 GHz frequency region. The technique of fixing undeterminable distortion constants to the corresponding values of the ground vibrational state for fundamental states and to extrapolated values for overtone and combination states was employed. The experimentally determined rotational, centrifugal distortion, and vibration-rotation interaction constants are reasonably well-predicted by computational methods (B3LYP/6-311+G(2d,p)). For the chlorine isotopologues, the changes in rotational and quartic distortion constants upon vibrational excitation are quite similar, indicating that it is possible to estimate the constants of a lower-abundance isotopologue’s excited vibrational state using the change in constant observed in the higher-abundance isotopologue. The changes in rotational and quartic distortion constants upon vibrational excitation are also quite similar between analogous vibrational states of 2-chloropyridine and chloropyrazine, despite their differences in molecular composition.

Millimeter-wave spectroscopy of the chlorine isotopologues of chloropyrazine and twenty-two of their vibrationally excited states

The measurement of over 24,000 new rotational transitions of chloropyrazine (C4H3N2Cl, μa = 1.55 D, μb = 0.14 D) in the 130–375 GHz frequency region enabled the assignments of the rotational spectra for the ground vibrational states of the [35Cl]- and [37Cl]-isotopologues and a combined total of 22 low-energy excited vibrational states. These vibrational states have been fit to sextic A-reduced Hamiltonians with low error (<0.05 MHz). These data allow for precise determination of the vibration-rotation interaction constants for the six lowest-energy fundamental vibrational modes (ν24, ν17, ν23, ν16, ν22, and ν15) of the [35Cl]-isotopologue and the four lowest-energy fundamental vibrational modes (ν24, ν17, ν23, and ν16) of the [37Cl]-isotopologue. In addition, many combination and overtone states were observed, bringing the total number of excited states to seventeen for the [35Cl]-isotopologue and five for the [37Cl]-isotopologue. The spectroscopic constants for the ground state, ν24, and ν17 for each isotopologue are very well determined (Nlines > 1400 for each least-squares fit). The vibration-rotation interaction constants experimentally determined for all observed fundamentals are generally in quite close agreement with their predicted (B3LYP/6-311+G(2d,p)) values.

Interference and phase mismatch effects in coherent triple sum frequency spectroscopy

Triply resonant sum frequency is a fully coherent method for multidimensional vibrational/electronic spectroscopy that creates cross-peaks between fundamental, overtone, and combination band vibrational states and electronic/vibronic states if they are coupled. It is also a methodology that cannot be phase matched. This paper explores the consequences of coherent interference and phase matching on the multidimensional spectra obtained in experiments performed in sample cells. Fringes appear in the multidimensional spectra and simulations show the character of the fringes depend on the refractive index dispersion, the thickness of different parts of the sample cell, the volume of the excitation region, and the relative third order susceptibility of the sample and the windows. It is suggested that the fringes provide a mechanism for determining the third order susceptibility and its multidimensional dispersion.

Nuclear spin/parity dependent spectroscopy and predissociation dynamics in vOH = 2 ← 0 overtone excited Ne-H2O clusters: Theory and experiment.

Vibrationally state selective overtone spectroscopy and state- and nuclear spin-dependent predissociation dynamics of weakly bound ortho- and para-Ne-H2O complexes (D0(ortho) = 34.66 cm-1 and D0(para) = 31.67 cm-1) are reported, based on near-infrared excitation of van der Waals cluster bands correlating with vOH = 2 ← 0 overtone transitions (|02-〉 and |02+〉) out of the ortho (101) and para (000) internal rotor states of the H2O moiety. Quantum theoretical calculations for nuclear motion on a high level potential energy surface [CCSD(T)/VnZf12 (n = 3, 4)], corrected for basis set superposition error and extrapolated to the complete basis set (CBS) limit, are employed to successfully predict and assign Π-Σ, Σ-Σ, and Σ-Π infrared bands in the spectra, where Σ or Π represent approximate projections of the body-fixed H2O angular momentum along the Ne-H2O internuclear axis. IR-UV pump-probe experimental capabilities permit real-time measurements of the vibrational predissociation dynamics, which indicate facile intramolecular vibrational energy transfer from the H2O vOH = 2 overtone vibrations into the VdWs (van der Waals) dissociation coordinate on the τprediss = 15-25 ns time scale. Whereas all predicted strong transitions in the ortho-Ne-H2O complexes are readily detected and assigned, vibrationally mediated photolysis spectra for the corresponding para-Ne-H2O bands are surprisingly absent despite ab initio predictions of Q-branch intensities with S/N > 20-40. Such behavior signals the presence of highly selective nuclear spin ortho-para predissociation dynamics in the upper state, for which we offer a simple mechanism based on Ne-atom mediated intramolecular vibrational relaxation in the H2O subunit (i.e., |02±〉 → {|01±〉; v2 = 2}), which is confirmed by the ab initio energy level predictions and the nascent OH rotational (N), spin orbit (Π1/2,3/2), and lambda doublet product distributions.

Torsion-rotation structure and quasi-symmetric-rotor behaviour for the CH3SH asymmetric CH3-bending and C-H stretching bands of E parentage

High-resolution Fourier transform spectra of the asymmetric methyl-bending and methyl-stretching bands of CH3SH have been recorded employing synchrotron radiation at the FIR beamline of the Canadian Light Source. Analysis of the torsion-rotation structure and relative intensities has revealed the novel feature that for both bend and stretch the in-plane and out-of-plane modes behave much like a Coriolis-coupled l-doublet pair originating from degenerate E modes of a symmetric top. As the axial angular momentum K increases, the energies of the coupled “l=±1” modes diverge linearly, with effective Coriolis ζ constants typical for symmetric tops. For the methyl-stretching states, separated at K=0 by only about 1cm−1, the assigned sub-bands follow a symmetric top Δ(K−l)=0 selection rule, with only ΔK=−1 transitions observed to the upper l=−1 in-plane A′ component and only ΔK=+1 transitions to the lower l=+1 out-of-plane A″ component. The K=0 separation of the CH3-bending states is larger at 9.1cm−1 with the l-ordering reversed. Here, both ΔK=+1 and ΔK=−1 transitions are seen for each l-component but with a large difference in relative intensity. Term values for the excited state levels have been fitted to J(J+1) power-series expansions to obtain substate origins. These have then been fitted to a Fourier model to characterize the torsion-K-rotation energy patterns. For both pairs of vibrational states, the torsional energies display the customary oscillatory behaviour as a function of K and have inverted torsional splittings relative to the ground state. The spectra show numerous perturbations, indicating local resonances with the underlying bath of high torsional levels and vibrational combination and overtone states. The overall structure of the two pairs of bands represents a new regime in which the vibrational energy separations, torsional splittings and shifts due to molecular asymmetry are all of the same order, creating a challenging and complex vibration-torsion-rotation coupling environment.

Multi-mode heterodyned 5th-order infrared spectroscopy.

Fifth-order multidimensional infrared spectroscopy with heterodyned detection was carried out in the three-beam dual-frequency configuration. Numerous 5th-order cross peaks were detected for the 4-azidobutyrate-N-hydroxysuccinimide ester compound in solution involving several vibrational modes ranging in frequency from 1045 to 2100 cm-1. Cross peaks involving overtones (2X/Z) and combination bands (XY/Z) among the tags, modes X and Y excited by the first two mid-IR laser pulses, and the reporter, modes Z excited by the third laser pulse, were acquired and the factors affecting the amplitude of 5th-order cross peaks are discussed. The 5th-order cross peaks were detected among modes that are spatially close (a few bonds apart) as well as for modes spatially separated by ca. 12 Å (eight bonds apart). In both cases, the waiting time dependences for the 3rd and 5th order cross peaks were found to be different. In particular, the waiting time at which the cross-peak maximum is reached, the decay time, and the value of a plateau at large waiting times were all differing strongly. The differences are explained by reduced sensitivity of the 5th-order signals to modes coupled weakly to the reporter mode and different relaxation dynamics involving overtone state of the tag. The ability of the 5th-order peaks to single out the modes coupled strongly to the reporter can help identifying specific energy relaxation and transport pathways, which will be useful for understanding energy transport dynamics in molecules. The absorptive 5th-order cross peaks were constructed which report on three-point correlation functions. It is shown that in addition to the triple-frequency correlation functions, a correlation of the frequencies with the mode coupling (anharmonicity) can be naturally measured by the 5th-order spectroscopy. The current limit for detecting 5th-order signals was estimated at the level of 1 × 10-3 in reduced anharmonicity, which is determined by the corresponding two-state anharmonicity divided by the reporter mode spectral width. Given the simplicity of recording the 5th-order cross peaks in the three-beam configuration, the approach carries a potential for a broad use.

Theoretical study of a positron-attachment to vibrational excited states for non-polar carbon disulfide molecule

We theoretically analyzed a positron affinity (PA), which is the binding energy of a positron, of the non-polar carbon disulfide (CS2) molecule at vibrational excited states to elucidate the effect of molecular vibrations on the binding of positron to the molecule. Using the configuration interaction method of the multi-component molecular orbital theory and anharmonic vibrational state analysis with vibrational quantum Monte Carlo technique, the vibrational averaged PA values are calculated as 0.39, 2.03, and 5.02 meV for the ground state, fundamental tone, and overtone states of asymmetric stretching mode, respectively. The PA value of CS2 molecule is found to be enhanced by the vibrational excitation of only asymmetric stretching mode compared to the value at the vibrational ground state. With the linear regression analysis, we have confirmed that such enhancement of vibrational averaged PA values mainly arises from the increment of molecular permanent dipole moment due to the vibrational excitations of the asymmetric stretching mode.

Interpretation of the vibrational energy level structure of the astructural molecular ion H5 (+) and all of its deuterated isotopomers.

Variational nuclear motion computations, employing an exact kinetic energy operator and two different potential energy surfaces, are performed to study the first 60 vibrational states of the molecular ion H5 (+)≡ [H2-H-H2](+) and all of its deuterated isotopologues and isotopomers, altogether 12 species. Detailed investigation of the vibrational wavefunctions mostly results in physically intuitive labels not only for the fundamentals but also for the overtone and combination states computed. The torsional motion associated with the left and right diatomics appears to be well separated from the other vibrational degrees of freedom for all species. The unusual structure of the higher-lying bending states and the heavy mixing of the internal motions is partly due to the astructural character of all these molecular ions. The existence of distinct isotopomers in the H5-nDn (+), n = 1-4 cases, in the energy range studied, is confirmed. Two rules determine the stability order of the isotopomers: first, when possible, H prefers to stay in the middle of the ions rather than at the sides, and, second, the isotopomer with a homonuclear diatomic at the side is always lower in energy. The large number of precise vibrational energies of the present study, as well as the detailed assignment of the states, should serve as benchmarks for future studies by more approximate nuclear-motion treatments, such as diffusion Monte Carlo and multiconfiguration time-dependent Hartree.

Theoretical analysis of positron-attachment to small molecules: Vibrational enhancement of positron affinities

SynopsisTo theoretically demonstrate the binding of a positron to small polarized molecules, we have calculated the vibrational averaged positron affinity (PA) values for small molecules with multi-component quantum Monte Carlo and molecular orbital methods. Our most accurate prediction of the vibrational averaged PA values at the fundamental and overtone states for formaldehyde molecule are 31 and 36 meV, respectively.

Hydrogen Bonding Induced Enhancement of Fermi Resonances: Ultrafast Vibrational Energy Flow Dynamics in Aniline-d5

With hydrogen bonding of the amino group of aniline-d5 we can identify the roles of Fermi enhanced combination and overtone states in intramolecular vibrational re-distribution (IVR) pathways for N-H stretching excitations. Using linear Fourier transform infrared (FT-IR) spectroscopy, ultrafast one- and two-color IR-pump-IR-probe spectroscopy, and femtosecond two-dimensional IR spectroscopy, we can identify the primary accepting modes for N-H stretching excitations. In particular, a key role is played by the δ(NH2) bending degree of freedom, either via its δ = 2 overtone state or via a combination state with the ν(C═C) ring stretching mode. No significant transient population in these Fermi enhanced combination/overtone states can be observed, a consequence of similar decay rates of these Fermi enhanced combination/overtone states and of the N-H stretching states. A similar magnitude of the transient response of the two fingerprint modes regardless of direct excitation of the Fermi enhanced combination/overtone levels or of the N-H stretching states suggests an underlying coupling mechanism facilitating common IVR pathways. This mechanism is expected to be of general importance for other organic compounds with hydrogen-bonded amino groups, including DNA bases.

Theoretical prediction of the binding of a positron to a formaldehyde molecule using a first-principles calculation

The vertical positron affinity (PA) value at the equilibrium position of a formaldehyde molecule is predicted as +25(3) meV with the highly accurate quantum Monte Carlo method. Applying anharmonic vibrational analysis, we have found that the vibrational excitation of the C = O stretching mode drastically enhances the PA value, due to the increment of the molecular dipole moment along this mode. Our predictions of the vibrational averaged PA values at the fundamental and overtone states are 31 and 36 meV, respectively, which strongly supports the conclusion that a positron can bind to formaldehyde.

Theoretical investigation of the binding of a positron to vibrational excited states of hydrogen cyanide molecule

We theoretically analyzed positron affinities (PA) of hydrogen cyanide (HCN) molecule at vibrational excited states to elucidate the effect of molecular vibrations on the binding of a positron to the molecule. Using the configuration interaction method in the multi-component molecular orbital theory and anharmonic vibrational state analysis with the variational Monte Carlo technique, we found that the vibrational excitations of the CN and CH stretching modes enhance the PA value compared to that of the vibrational ground state, whereas the excitation of bending mode deenhances it. The largest PA enhancement is found at the excited states of the CH stretching mode; the PA values are 43.02 (1) and 46.34 (2) meV for the fundamental tone and overtone states, respectively. With the linear regression analysis, we confirmed that the PA variation of HCN molecule at each vibrational state arises from the variation of permanent dipole moment and dipole-polarizability due to each vibrational excitation.

Laser-induced grating spectroscopy of highly excited overtone and combination vibrational states of methane

Laser-induced grating spectroscopy is applied to investigate weak overtone and combination absorption bands of methane. The rotationally-resolved spectra of transitions to the polyads Pn, n=8–10, 12 recorded at pressures ranging from 0.2 to 4bar and temperatures 298K and 124–130K cover a broad interval between 891 and 618nm (11220–16180cm−1) and contain a significant amount of prominent narrow lines which correspond to stationary rovibrational states. The absorption cross sections of the weakest detected methane transitions are estimated to be as small as ≈10−26cm2molecule−1. The technique is suitable to provide data for the development and verification of new models describing the complex energy structure of highly vibrationally excited molecules.

Raman spectroscopy and high-overtone driven isomerization of glyoxylic acid in solid argon

High-overtone induced chemistry of glyoxylic acid isolated in a low-temperature argon matrix was investigated using Raman spectroscopy. The Raman spectra of two most stable conformers of glyoxylic acid are presented. Upon excitation in high overtone vibrational bands by 532 nm irradiation of the lowest energy conformer most abundant in neat deposited sample, the isomerization of glyoxylic acid was observed. The process most plausible proceeds via absorption into the fifth vibrational overtone state of the OH group or its combination with the torsional vibrational transition. The assignment of the fundamental vibrational spectra was assisted by quantum chemical harmonic and anharmonic vibrational calculations.

Variational Calculations of Vibrational Energies and IR Spectra of trans- and cis-HOCO Using New ab Initio Potential Energy and Dipole Moment Surfaces

We report ab initio potential energy and dipole moment surfaces that span the regions describing the minima of trans- and cis-HOCO and the barrier separating them. We use the new potential in three types of variational calculations of the vibrational eigenstates, for zero total angular momentum. Two use the code MULTIMODE (MM) in the so-called single-reference and reaction path versions. The third uses the exact Hamiltonian in diatom-diatom Jacobi coordinates. The single-reference version of MM is limited to a description of states that are localized at each minimum separately, whereas the reaction-path version and the Jacobi approach describe localized and delocalized states. The vibrational IR spectrum for zero total angular momentum is also reported for the trans and cis fundamentals and selected overtone and combination states with significant oscillator strength.

Variational study on the vibrational level structure and vibrational level mixing of highly vibrationally excited S0 D2CO

We perform converged high precision variational calculations to determine the frequencies of a large number of vibrational levels in S0 D2CO, extending from low to very high excess vibrational energies. For the calculations we use our specific vibrational method (recently employed for studies on H2CO), consisting of a combination of a search/selection algorithm and a Lanczos iteration procedure. Using the same method we perform large scale converged calculations on the vibrational level spectral structure and fragmentation at selected highly excited overtone states, up to excess vibrational energies of ∼17000cm−1, in order to study the characteristics of intramolecular vibrational redistribution (IVR), vibrational level density and mode selectivity.

High-resolution infrared spectroscopy of diacetylene below 1000 cm−1

The infrared spectrum of diacetylene has been recorded at high resolution between 500 and 1000 cm−1 by Fourier transform spectroscopy. More than 1200 transitions were assigned to five bands: the ν8 fundamental, the ν7 + ν9 combination, and the ν3 − ν9 difference bands as well as the most intense ν8 + ν9 − ν9 and ν7 + 2ν9 − ν9 hot bands. The data were analysed together with the previously recorded millimetre-wave lines for the ν8 − ν6 and ν8 + ν9 − (ν6 + ν9) difference bands. Rotational and vibrational l-type resonances, together with the cubic anharmonic interactions which couple the v 3 = 1 stretching state with the v 8 = v 9 = 1 combination and v 7 = 2 overtone states, have been considered in the least-squares fits to the observed wavenumbers in order to derive reliable spectroscopic parameters. The spectral analyses deliver very precise B 0 and D 0 parameters and experimental values of the anharmonicity constants for the bending–bending combination states of diacetylene below 1000 cm−1.

Influence of initial parent vibrational excitation in promoting two-photon absorption in HOOH and CH 3OOH

Two-photon dissociation of vibrationally excited HOOH and CH 3OOH is demonstrated from their 2 ν OH stretching overtone state using ∼205 nm light. Initial vibrational excitation appears to be a prerequisite for accessing the two-photon excited dissociative electronic state in these hydroperoxides, most likely due to the ability of vibrational excitation to improve the Franck–Condon factor. The hydroperoxide excited state accessed through the vibrationally mediated photodissociation process generates OH fragments in their A 2Σ + electronic excited state. Detection of the OH(A 2Σ +) fluorescence as a function of the vibrational excitation laser allows recording of the 2 ν CH overtone action spectra for CH 3OOH with high sensitivity.

Anharmonic overtone and combination states of glycine and two model peptides examined by vibrational self-consistent field theory

In this paper, the application of the vibrational self-consistent field (VSCF) and correction-corrected VSCF methods for calculating anharmonic parameters, including transition frequency, transition intensity and dipole, and vibrational anharmonicity of 3N-6 normal modes for formamide, glycine, N-methylacetamide and their deuterated derivatives are explored mainly at the level of density functional theory. The computed fundamental anharmonic frequencies are found to be in reasonable agreement with experimental results. Diagonal anharmonicities of the second overtone states were examined for multiple normal modes, whose magnitudes were found to correlate well with those of the first overtone states in the three small molecules. The results show that the VSCF-based approach can be utilized to predict anharmonic parameters of higher vibrational states that are essential to understanding multi-pulse infrared nonlinear experiments of peptides.