Overtone Region Research Articles

Resonance coupling in the fourth OH-stretching overtone spectrum of formic acid.

The room-temperature vibrational overtone spectra of the formic acid isotopomers HCOOH and DCOOH have been recorded in the third and fourth OH-stretching overtone regions with intracavity laser photoacoustic spectroscopy. Resonance coupling between the OH- and CH-stretching vibrations in HCOOH is clearly identified in the fourth overtone region. This is an example of strong coupling across bonds. In the third overtone region, no resonance is observed. Vibrational energies and intensities of the OH- and CH-stretching overtones and combination bands have been calculated with an anharmonic oscillator local mode model. The pure OH-stretching bright state carries almost all the intensity prior to resonance coupling.

A Comparative Study of Overtone CH-Stretching Vibrational Circular Dichroism Spectra of Fenchone and Camphor

Near-infrared vibrational circular dichroism (VCD) spectra in the region 1500−850 nm are recorded for both enantiomers of camphor and fenchone using a home-built dispersive instrument. Two CH-stretching overtone regions and two CH-stretching/HCH-bending combination regions are investigated. The near-infrared VCD data are examined and compared with the corresponding previously published data of (3R)-(+)-methylcyclopentanone and (3R)-(+)-methylcyclohexanone. The absorption spectra of both the fundamental and overtone regions are also considered, and the fundamental CH-stretching VCD data are taken into account, where possible. An empirical assignment of the spectra is first considered, with special regard to the second CH-stretching overtone region. This region is then investigated by a heuristic approach based on DFT calculations, using the hybrid functional B3LYP at the 6-31G** basis set level. Subsequently rotational strengths have been evaluated by use of a classical model based on the contribution of p...

Intramolecular Dynamics in the Photofragmentation of Initially Vibrationally Excited CH2Cl2

The intramolecular dynamics of dichloromethane was studied by applying vibrationally mediated photodissociation and photoacoustic spectroscopy. The parent species were initially excited to the second, third, and fourth C-H stretch overtone regions and subsequently photodissociated by ∼235 nm photons that also tagged the ground, Cl( 2 P 3 / 2 ) [Cl], and spin-orbit-excited, Cl( 2 P 1 / 2 ) [Cl*], state photofragments via resonantly enhanced multiphoton ionization. The Cl and Cl* action spectra revealed enhancement of their yield as a result of rovibrational excitation with a Cl*/Cl ratio of about ½. The action and photoacoustic spectra manifested a multiple-peak structure, which in terms of normal- and local-mode models is shown to be related to overtones of C-H stretches or combination bands of C-H stretches and bends. The measured time-of-flight profiles together with the determined Cl*/Cl branching ratios suggest fast dissociation and involvement of upper states of A' and A symmetry, mixing via curve crossing and releasing both Cl and Cl*. The determined Cl*/Cl ratios in photodissociation of vibrationally excited CH 2 Cl 2 are higher than those obtained previously in 193 nm photodissociation of the vibrationless ground state, implying higher nonadiabaticity for the former.

First stretching overtone of BiH3: an extreme local-mode case for XH3-type molecule?

The first stretching overtone region of short-lived, formerly inaccessible BiH3 near 3405 cm(-1) has been measured by Fourier-transform infrared spectroscopy with a resolution of 0.0066 cm(-1). Only the 2nu1(A1)/nu1+nu3(E) band system has been observed. Rotational analysis, with transitions reaching J'max=14, has revealed almost perfect local-mode behavior for the upper states denoted as (200A1/E) in the local-mode notation. Ratios of vibration-rotation interaction parameters q(eff)/alpha(eff)(BB) and r(eff)/alpha(eff)(BC), and the appropriate rotational constant differences, are in good agreement with theoretical local-mode limit values. A simple stretching vibrational model reproduces the observed vibrational term values well, and the potential parameters obtained are close to true values.

Spectroscopic characterization of HOONO and its binding energy via infrared action spectroscopy

The trans–perp conformer of peroxynitrous acid is identified by action spectroscopy in the OH overtone region and assigned by simulating its band structure using a transition moment and rotational constants from ab initio theory. The highest observed OH product state sets an upper limit for the O–O bond energy of tp-HOONO of 16.8 kcal/mol.

Infrared and Ultraviolet Spectroscopy of Jet-Cooled ortho-, meta-, and para-Diethynylbenzene

The vibronic spectroscopy of ortho-, meta-, and para-diethynylbenzene (oDEB, mDEB, and pDEB) was studied by two-color resonant two photon ionization (R2PI). The symmetry allowed S0−S1 origins of oDEB, mDEB, and pDEB were located at 33 515, 33 806, and 34 255 cm-1, respectively, with the vibronic structure extending about 2000 cm-1 above the origin in oDEB and mDEB, but more than 3000 cm-1 in pDEB. Major peaks in each spectrum were attributed to vibronically induced bands, indicating strong coupling of the S1 state to the S2 state. Ground-state infrared spectra in the C−H stretch region (3000−3360 cm-1) were obtained using resonant ion-dip infrared spectroscopy (RIDIRS). In all three isomers, the acetylenic C−H stretch fundamental was split by Fermi resonance with a combination band composed of the C⋮C stretch and two quanta of the C⋮CH bend. This Fermi resonance was detuned in the overtone region of pDEB, which showed a single peak at 6556 cm-1. Infrared spectra were also recorded in the excited electroni...

Calculation of spectroscopic parameters and vibrational overtones of methanol

A high-level quartic ab initio potential energy surface of methanol has been used to calculate spectroscopic constants of the 12CH3OH molecule. These include coefficients of quartic anharmonic resonance terms, Darling-Dennison constants, for stretching states. A model expressed in terms of dimensionless normal coordinates has been employed in the calculation of O—H and C—H stretching vibrational states in high-overtone regions. Both cubic Fermi and quartic Darling-Dennison anharmonic coupling terms have been included in the model in order to take into account strong resonances between different states. The nonlinear least-squares method has been used to optimize some of the model parameters employing experimental term values of 12CH3OH as data. Vibrational assignments are suggested for the first C—H stretching overtone region.

Infrared spectra of CH3F(ortho-H2)n clusters in solid parahydrogen

The formation of CH3F(ortho-H2)n clusters in rapid vapor deposited solid molecular hydrogen containing low concentrations of CH3F and ortho-H2 has been investigated using high-resolution Fourier transform infrared spectroscopy of the C–F stretching mode (ν3) of the CH3F chromophore. Distributions of CH3F(ortho-H2)n clusters ranging in size from n=0 to n=12 are synthesized in para-H2 crystals by systematically varying the ortho-H2 concentration between 100 to 20 000 parts per million. The rotational motion of CH3F is quenched in solid para-H2; this simplifies the spectrum such that a single sharp transition is observed for each cluster. In the fundamental ν3 region, the spectrum consists of a series of well-separated peaks shifted toward lower energy with increasing numbers of nearest neighbor orthohydrogen molecules. The CH3F(ortho-H2)n clusters are also investigated in the ν3 overtone region and for analogous transitions of the CD3F(ortho-H2)n. Theoretical calculations based on an electrostatic interaction potential qualitatively reproduce the measured vibrational and isotopomer dependence of the spectra.

Infrared action spectroscopy and time-resolved dynamics of the OD–CO reactant complex

The infrared action spectrum of the linear OD–CO reactant complex has been recorded in the OD overtone region near 1.9 μm using an infrared pump-ultraviolet probe technique. The pure overtone band of OD–CO (2νOD) is observed at 5148.6 cm−1 and combination bands involving the simultaneous excitation of OD stretch and D-atom bend are identified 160.0 and 191.2 cm−1 to higher energy. Band assignments and spectroscopic constants are derived from the rotationally resolved structure of the spectra. The change in the ground state rotational constant upon deuteration demonstrates that the H/D-atom of the hydroxyl radical points toward CO in the OH/D-CO complex. Direct time-domain measurements yield a lifetime of 37(4) ns for OD–CO (2νOD) prior to decay via inelastic scattering or chemical reaction. This is significantly longer than the laser-limited lifetime of ⩽5 ns observed for OH–CO (2νOH), and is attributed in part to the closing of a near-resonant vibration to vibration energy transfer channel upon deuteration. Vibrational predissociation of OD–CO (2νOD) proceeds by a vibration to rotation and/or translation mechanism that yields highly rotationally excited OD (v=1) fragments. Intermolecular D-atom bend excitation, which drives the structural transformation from the reactant complex to the transition state for reaction, results in a dramatic shortening of the lifetime to ⩽6 ns (laser-limited). Excitation of the D-atom bend also supplies sufficient energy to reopen the near-resonant vibrational energy transfer channel, resulting in minimal rotational excitation of the OD (v=1) fragments. Finally, a ground state binding energy for OD–CO of D0⩽456 cm−1 is established from the OD (v=1) product state distribution.

Effects of pair resonant dipole-dipole interactions on the formation of band profiles in infrared absorption spectra

Resonant dipole-dipole interactions are shown to play a crucial role in forming the profiles of several bands in the fundamental and overtone regions of the vibrational spectrum for molecules with a relatively large first derivative of the dipole moment (P′>0.3 D). Analytic expressions are derived for the probability density of the matrix element for the interaction of one-and two-dimensional oscillators. We computed the band profiles in the infrared absorption spectrum for a pair of molecules at a fixed distance R ∼ 5 A, typical of condensed media. Comparison is made with experimental data for low-temperature condensed systems: matrices and solutions in liquefied gases.

Two-vibron bound states lifetime in a one-dimensional molecular lattice coupled to acoustic phonons

The lifetime of two-vibron bound states in the overtone region of a one-dimensional anharmonic molecular lattice is investigated. The anharmonicity, introduced within an attractive Hubbard Hamiltonian for bosons, is responsible for the formation of bound states which belong to a finite linewidth band located below the continuum of two-vibron free states. The decay of these bound states into either bound or free states is described by considering the coupling between the vibrons and a thermal bath formed by a set of low-frequency acoustic phonons. The relaxation rate is expressed in terms of the spectral distribution of the vibron/phonon coupling and of the two-vibron Green operator which is calculated exactly by using the number states method. The behavior of the two-vibron bound states relaxation rate is analyzed with a special emphasis on the influence of the anharmonicity. It is shown that the rate exhibits two distinct regimes depending on the thermal bath dimension. When the bath dimension is equal to unity, the rate increases with the anharmonicity and the decay of the two-vibron bound states into the other bound states appears as the main contribution of the rate. By contrast, when the bath dimension is equal to 2 and 3, the rate decreases as the anharmonicity increases, indicating that the two-vibron bound states decay into the two-vibron free states continuum.

Vibrational overtone spectra of metallocenes: effect of the coordinating metal on the CH bond lengths

The first through third overtone spectra of ferrocene, ruthenocene, nickelocene, cobaltocene, dicyclopentadienyl magnesium and sodium cyclopentadienyl are examined with particular attention to the CH stretching of the cyclopentadienyl. Using semi-empirical correlations between CH bond length and CH stretching frequencies in each overtone region, we have determined that the type of metal atom within a metallocene complex has little effect on the CH bond length in the cyclopentadienyl. The only exception is cobaltocene where there is evidence that the Jahn-Teller effect results in several different CH bond lengths. Evidence that bis(cyclopentadienyl) magnesium is not ionic has been observed.

Decay dynamics of the vibrationally activated OH–CO reactant complex

The decay dynamics of the OH–CO reactant complex have been examined following infrared excitation in the OH overtone region using various IR pump–UV probe methods. The time scale for overall decay of the OH–CO (2vOH) complex has been bracketed between 0.19 and 5 ns through linewidth and direct time-domain measurements. The inelastically scattered OH (v=1) fragments exhibit a bimodal internal energy distribution, which reveals that vibrational predissociation proceeds through two pathways. The dominant inelastic decay channel involves vibrational energy transfer from OH to CO with little excess energy remaining for rotational excitation of the OH fragment, while a slower secondary channel releases most of the excess energy as OH rotational excitation. Intermolecular bending excitation of the OH–CO complex through combination bands results in increased rotational excitation of the OH fragments. The most probable OH product states display a strong lambda-doublet preference indicating that the singly occupied pπ orbital of OH is aligned perpendicular to the OH rotation plane following vibrational predissociation of the complex. These product states also minimize the translational recoil of the fragments and maximize the rotational angular momentum of the OH fragment. Abrupt cutoffs in the OH (v=1) fragment internal energy distributions are utilized to determine an upper limit for the ground state binding energy of OH–CO, D0⩽410 cm−1, which is in good accord with ab initio predictions. Finally, a comparison of infrared band intensities obtained using action and depletion detection methods suggests that geared bend and H-atom bend excitation may promote reactive decay of the OH–CO reactant complex.

Intermolecular vibrations of the hydrogen bonded OH–CO reactant complex

Intermolecular vibrations of the linear OH–CO reactant complex have been observed as combination bands in the OH overtone region using infrared action spectroscopy. Rotational analyses and simulations of the band structures have been carried out for transitions to geared bend, excited spin–orbit, and H-atom bend states with 50–250 cm−1 of intermolecular excitation. The projection quantum number associated with each of these upper states is identified through the intensity profile of the band contour, missing rotational lines, and/or parity splitting of individual rotational lines. Intermolecular states with projection quantum numbers P=1/2 and 5/2 are observed for each of the two bending modes, arising from coupling of the unquenched angular momentum of OH with the vibrational angular momentum associated with the bending motion of the complex. An additional P=1/2 state is attributed to spin–orbit excitation, which shifts to higher energy than in free OH and gains infrared transition strength through the spin-decoupling interaction. The intermolecular energy level pattern is also examined in the context of the Renner-Teller interaction and spin–orbit coupling. The intermolecular bends of the OH–CO complex are of special interest because they probe portions of the reaction path leading to trans-HOCO formation.

Quantitative Analysis of Epoxy Resin Cure Reaction: A Study by Near-Infrared Spectroscopy

Near-infrared spectroscopy was used to quantify the cure reaction of 4,4′-methylene- bis-(2,6-diethylaniline) (MDEA)–epoxy resins (E/A = 1.4) carried out at 72 and 160 °C. The absorption bands of the functional groups of interest in MDEA–epoxy resins are assigned according to the literature. A new assignment at 6580 cm−1 is also proposed for the secondary amine: it was supported by a synthesized model compound. Two different spectrum treatments were proposed. The first one is based only on a normalization at 4610–4620 cm−1, while the second one needs the subtraction of the normalized spectrum of a post-cure sample. To follow the curing process, amines and epoxy were studied at the same time in the combination and the overtone regions. The results are compared. In the combination region, quantitative results are obtained from absorbance measurements, while in the overtone region spectrum decompositions and area measurements are necessary. Complementary and reliable information are so obtained and allow us to calculate conversions of epoxide and amine I and concentrations in amine II, amine III, hydoxyl groups, and ether links. Kinetics are also established. The curing process mechanism is at last discussed for both curing temperatures.

Dipole and rotational strengths for overtone transitions of a C2-symmetry HCCH molecular fragment using Van Vleck perturbation theory

Contact transformation theory up to second order is employed to treat CH-stretching overtone transitions and to calculate dipole and rotational strengths. A general Hamiltonian describing two interacting CH-stretching oscillators is considered, and the Darling–Dennison resonance is appropriately taken into account. The two CH bonds are supposed to be dissymmetrically disposed, so as to represent a chiral HCCH fragment, endowed with C2 symmetry. Analytical expressions of transition moments and dipole and rotational strengths are given in the hypothesis of general electric and magnetic dipole moments with quadratic dependence on coordinates and momenta. Dipole and rotational strengths are then calculated together with frequencies for the fundamental and first three overtone regions in the simplifying hypothesis of the valence optical approach on the coupled-oscillator framework. Simplified analytical expressions thereof in the relevant parameters are presented.

Vibrationally Mediated Photodissociation of Jet-Cooled CH3CF2Cl: A Probe of Energy Flow and Bond Breaking Dynamics

A double resonance scheme was employed to photodissociate jet-cooled CH3CF2Cl in a time-of-flight mass spectrometer. First, the molecule was promoted to the second (3νCH) or third (4νCH) methyl overtone by direct IR excitation. Subsequently, a UV laser beam at ∼235 nm was used to both dissociate the molecule and tag the Cl 2P3/2 [Cl] and Cl 2P1/2 [Cl*] photofragments by (2 + 1) resonantly enhanced multiphoton ionization. The photofragment action spectra revealed multiple peak structures in both overtone regions, attributed to couplings of the C−H stretches and to methyl stretch−deformation Fermi resonances. The cooling of the samples afforded narrowing of the peaks, relative to room temperature photoacoustic spectra, due to the reduced rotational and vibrational congestion, and enabled observation of a new splitting in the high frequency peak of the second overtone. This splitting apparently resulted from a local resonance of the mixed stretch−deformation state with a close lying dark state. The time scales for vibrational energy redistribution in the local resonance and between the mixed stretch−deformations were evaluated. These time scales and the measured Cl*/Cl branching ratios were compared to those of other hydrohalocarbon compounds. The Cl*/Cl ratio was also compared to that of the nearly isoenergetic vibrationless ground state 193 nm photodissociation and found to be different, demonstrating the effect of vibrational pre-excitation on the photodissociation dynamics.

Real-Time Observation of Intra- and Intermolecular Vibrational Energy Flow of Selectively Excited Alkyl Iodides in Solution: The Effect of Chemical Substitution

Intramolecular vibrational energy redistribution (IVR) and intermolecular vibrational energy transfer (VET) of two alkyl iodides (CF3CH2I and CH3CH2I) selectively excited in the two quanta overtone region of the CH stretch vibration (υCH = 2) were measured in real time in solution. In this study, we have focused on the effect of chemical substitution on the mechanisms and time scales of IVR and VET of this family of molecules. With a simple model, we have obtained global IVR and VET rate coefficients for both molecules. The magnitude and the variation of the relaxation rates upon chemical substitution provide evidence for a survival of hierarchical IVR in these solvated molecules, which is governed by specific low-order intramolecular interactions and which can be rationalized with a simple low-order coupling model. At the same time, the general assumption that VET is simply dominated by the lowest-frequency modes in a molecule is not supported.

Calculated OH-Stretching Vibrational Transitions of the Water−Nitric Acid Complex

We have calculated fundamental and overtone OH-stretching vibrational band frequencies and intensities of the water−nitric acid complex. The calculations use the simple harmonically coupled anharmonic oscillator (HCAO) local-mode model with local-mode parameters obtained from scaled ab initio calculations and ab initio-calculated dipole moment functions. The ab initio calculations were performed at the HF, B3LYP, and QCISD levels of theory predominantly with the 6-311++G(2d,2p) basis set. We have compared our results for the water−nitric acid complex with results for the water dimer and the nitric acid and water molecules. The results show that the water−nitric acid complex is more strongly bound, and the changes in spectroscopic properties compared to the individual molecules are more significant than those for the water dimer. The total OH-stretching intensity of the water−nitric acid complex is significantly higher than the sum of the intensities for the individual molecules in the fundamental and higher overtone regions. The transitions associated with the hydrogen-bonded OH bond show a very large red shift compared to the OH-stretching transitions of the nitric acid molecule. These red-shifted bands provide likely spectral regions for the detection of the water−nitric acid complex in the near-infrared. The effect of the water−nitric acid complex on atmospheric OH radial productions and absorption of solar radiation is discussed.

OD–N 2 : Infrared spectroscopy, potential anisotropy, and predissociation dynamics from infared-ultraviolet double resonance studies

The infrared spectrum of the linear OD–N2 complex has been recorded in the OD overtone region near 1.9 μm using an infrared-ultraviolet double resonance technique. The pure overtone band of OD–N2(2νOD) was observed at 5173.99(1) cm−1, and combination bands involving the simultaneous excitation of OD stretch and geared bend were identified at 5209.02(1) cm−1 and 5214.59(2) cm−1. Assignments and spectroscopic constants have been derived from the rotationally resolved structure of each band, which are in good accord with model calculations based on an electrostatic interaction potential. Direct time–domain measurements yielded a vibrational predissociation lifetime of 150±16 ns for OD–N2 (2νOD) and a three fold decrease in lifetime upon intermolecular excitation of the lower-energy geared bending state. The OD (v=1) fragments of vibrational predissociation were found to be highly rotationally excited, indicating that predissociation proceeds by vibrational to rotational/translational energy transfer. The results obtained for OD–N2 are compared with an analogous study of OH–N2 [Marshall et al., J. Chem. Phys. 114, 7001 (2001)], revealing insights on the potential anisotropy along the geared bending coordinate and a change in the vibrational predissociation mechanism.