OH stretch overtone spectroscopy and transition dipole alignment of HOD

Experimental evidence of Fermi resonances in isotopically dilute water from ultrafast broadband IR spectroscopy

The vibrational dynamics of liquid water, which result from a complex interplay between internal molecular vibrations and the fluctuating hydrogen bond network, are fundamental to many physicochemical and biological processes. Using a new ultrafast broadband mid-infrared light source with over 2000 cm(-1) of bandwidth, we performed ultrafast time-resolved infrared spectroscopy to study the vibrational couplings and relaxation dynamics of the stretching and bending vibrations of the mixed isotopologue, HOD, in D2O. Analysis of cross-peaks and induced absorptions in the two-dimensional infrared spectrum and transient absorption spectrum shows that the hydroxyl stretch of HOD is coupled to the HOD bending mode via Fermi resonance, with a 70° angle between their transition dipole moments. We see that HOD is also anharmonically coupled to the D2O solvent modes. From transient absorption spectra, we conclude that vibrational relaxation occurs through a number of paths. The strongly hydrogen-bonded OH oscillators have the highest propensity to relax through the bending mode, while the weakly hydrogen bonded oscillators relax through other modes.

Quantification of transition dipole strengths using 1D and 2D spectroscopy for the identification of molecular structures via exciton delocalization: application to α-helices

Vibrational and electronic transition dipole strengths are often good probes of molecular structures, especially in excitonically coupled systems of chromophores. One cannot determine transition dipole strengths using linear spectroscopy unless the concentration is known, which in many cases it is not. In this paper, we report a simple method for measuring transition dipole moments from linear absorption and 2D IR spectra that does not require knowledge of concentrations. Our method is tested on several model compounds and applied to the amide I(') band of a polypeptide in its random coil and α-helical conformation as modulated by the solution temperature. It is often difficult to confidently assign polypeptide and protein secondary structures to random coil or α-helix by linear spectroscopy alone, because they absorb in the same frequency range. We find that the transition dipole strength of the random coil state is 0.12 ± 0.013 D(2), which is similar to a single peptide unit, indicating that the vibrational mode of random coil is localized on a single peptide unit. In an α-helix, the lower bound of transition dipole strength is 0.26 ± 0.03 D(2). When taking into account the angle of the amide I(') transition dipole vector with respect to the helix axis, our measurements indicate that the amide I(') vibrational mode is delocalized across a minimum of 3.5 residues in an α-helix. Thus, one can confidently assign secondary structure based on exciton delocalization through its effect on the transition dipole strength. Our method will be especially useful for kinetically evolving systems, systems with overlapping molecular conformations, and other situations in which concentrations are difficult to determine.

Vibrational overtone spectroscopy of phenol and its deuterated isotopomers

We have measured the OH- and OD-stretching fundamental and overtone spectra of phenol and its deuterated isotopomers under jet-cooled conditions using nonresonant ionization detection spectroscopy and vapor-phase infrared (IR) and near-infrared (NIR) spectra at room temperature using conventional and photoacoustic spectroscopy. The OH- and OD-stretching bands in the jet-cooled spectra are about 1-10 cm(-1) wide and generally show a few Lorentzian shaped peaks. The bands in the room-temperature spectra have widths of 20-30 cm(-1) and display clear rotational profiles. The band profiles in the jet-cooled spectra arise mostly from nonstatistical intramolecular vibrational redistribution (IVR) with specific coupling to "doorway" states, which are likely to involve CH- and CD-stretching vibrations. The transition dipole moment that determines the rotational structure is found to rotate significantly from the fundamental to the third overtone and is not directed along the OH(D) bond. We use these calculated transition dipole moments to simulate the rotational structure. We determine the rotational temperature in the jet-cooled spectra to be about 0.5 K. Anharmonic oscillator local mode calculations of frequencies and intensities of the OH- and OD-stretching transitions are compared with our measured results. The calculated intensities are in good agreement with the absolute intensities obtained from conventional spectroscopy and with the relative intensities obtained from the room-temperature laser spectroscopy.

Supersonically cooled hydronium ions in a slit-jet discharge: High-resolution infrared spectroscopy and tunneling dynamics of HD2O+

Jet-cooled high-resolution infrared spectra of partially deuterated hydronium ion (HD2O+) in the O-H stretch region (nu3 band) are obtained for the first time, exploiting the high ion densities, long absorption path lengths, and concentration modulation capabilities of the slit-jet discharge spectrometer. Least-squares analysis with a Watson asymmetric top Hamiltonian yields rovibrational constants and provides high level tests of ab initio molecular structure predictions. Transitions out of both the lower (nu3(+)<--0(+)) and the upper (nu3(-)<--0(-)) tunneling levels, as well as transitions across the tunneling gap (nu3(-)<--0(+)) are observed. The nu3(-)<--0(+) transitions in HD2O+ acquire oscillator strength by loss of D(3h) symmetry, and permit both ground-state-[27.0318(72) cm(-1)] and excited-state-[17.7612(54) cm(-1)]-tunneling splittings to be determined to spectroscopic precision from a single rovibrational band. The splittings and band origins calculated with recent high level ab initio six-dimensional potential surface predictions for H3O+ and isotopomers [X. C. Huang, S. Carter, and J. M. Bowman, J. Chem. Phys. 118, 5431 (2003); T. Rajamaki, A. Miani, and L. Halonen, J. Chem. Phys. 118, 10929 (2003)] are in very good agreement with the current experimental results.

Effective One-Dimensional Dipole Moment Function for the OH Stretching Overtone Spectra of Simple Acids and Alcohols

To gain insight on the absorption intensities, as well as the direction of the transition moment for the OH stretching vibration in alcohols and acids, we performed detailed analyses for nitric acid, acetic acid, methanol, tert-butyl alcohol, water, and OH radical. We obtained both the potential energy surface and the dipole moment function (DMF) by the B3LYP method and performed quantum mechanical vibrational calculation using the grid variational method based on the local mode model. In this work, we employed the sum rule of the absorption intensities for the one-dimensional (1-D) vibrational Hamiltonian to construct an effective 1-D DMF, which is responsible for the total sum of the overtone intensities. The direction of this effective DMF was found to be tilted away from the OH bond by about 30 degrees for the polyatomic molecules. The nonlinearity of the DMFs in the directions parallel and perpendicular to the OH bond is discussed to rationalize the tilting. Furthermore, we analyzed the effective 1-D DMFs with the vibrational wave function expansion method and derived the effective portion of the 1-D DMF that is responsible for the overtone transition moment.

Intracluster stereochemistry in van der Waals complexes: Steric effects in ultraviolet photodissociation of state-selected Ar–HOD/H2O

High-resolution IR-UV multiple resonance methods are employed to elucidate the photodissociation dynamics of quantum state-selected Ar-HOD and Ar-H(2)O van der Waals clusters. A single mode pulsed OPO operating in the region of the OH second overtone is used to prepare individual rovibrational states that are selectively photodissociated at specific excimer wavelengths. Subsequent fluorescence excitation of the resulting OH (OD) fragments yields dynamical information on the photofragmentation event and any resulting intracluster collisions. This technique is used to characterize spectroscopically the Pi(1(01)), nu(OH)=3<--Sigma(0(00)), v(OH)=0 overtone band of the Ar-HOD complex with an origin at 10648.27 cm(-1). The effects of Ar complexation on the dissociation dynamics are inferred by comparison of the OD photofragment quantum state distributions resulting from dissociation of single rovibrational states of the complex with those from isolated HOD photodissociation. The important role played by the initial internal state of the complex is demonstrated by comparison of the current Ar-HOD data with previously published results for the Ar-H(2)O Sigma(0(00))[03(-)> state. We interpret the dramatic differences in the dynamics of the two systems as manifestations of the nodal structure of the vibrational state in the parent complex and the way in which it governs the collision probability between the Ar atom and the escaping photofragments.

The 5V(OH) Overtone Transition of HDO

The absorption spectrum of HDO has been recorded by intracavity laser absorption spectroscopy in the 16 540-17 055 cm(-1) spectral region corresponding to the 5nu(3) band centered at 16 920 cm(-1). The (0 0 5) vibrational state is found to be mostly isolated from the nearby rovibrational states. The corresponding rovibrational transitions were analyzed and fitted in the frame of the effective rotational Hamiltonian model in Pade-Borel approximants form. The spectroscopic parameters retrieved from the fitting reproduce 100 of the 109 determined energy levels with the root-mean-square deviation of 0.0072 cm(-1), close to the experimental accuracy. From the integrated relative intensities of a- and b-type transitions, the angle between the transition moment and the OH bond is estimated to be 46.4 degrees. This value is consistent with an increasing tilt of the transition dipole moment, away from the OH bond, when the OH stretching is excited. The evolution of the orientation of the transition dipole moment versus the vibrational excitation is then compared for the OH and OD overtone bands. Copyright 2000 Academic Press.

Experimental and Ab Initio Studies of the HDO Absorption Spectrum in the 13 165-13 500 cm(-1) Spectral Region

The HDO absorption spectrum was recorded in the 13 165-13 500 cm(-1) spectral region by intracavity laser absorption spectroscopy. The spectrum (615 lines), dominated by the 2nu(2) + 3nu(3) and nu(1) + 3nu(3) bands, was assigned and modeled leading to the derivation of 196 accurate energy levels of the (103) and (023) vibrational states. Finally, 150 of these levels were reproduced by an effective Hamiltonian involving two vibrational dark states interacting with the (023) and (103) bright states. The rms deviation achieved by variation of 28 parameters is 0.05 cm(-1), compared to an averaged experimental uncertainty of 0.007 cm(-1), indicating the limit of validity of the effective Hamiltonian approach for HDO at high-vibrational excitation. The predictions of previous ab initio calculations of the HDO spectrum (H. Partridge and D. Schwenke, J. Chem. Phys. 106, 4618-4639 (1997)) were extensively used in the assignment process. The particular spectral region under consideration was used to test and discuss the improvements of new ab initio calculations recently performed on the basis of the same potential energy surface but with an improved dipole-moment surface. The improvements concern both the energy levels and the line intensities. In particular, the strong hybrid character of the nu(1) + 3nu(3) band is very well accounted for by the new ab initio calculations. Copyright 2000 Academic Press.

The 4V(OH) Absorption Spectrum of HDO

The absorption spectrum of HDO was recorded by intracavity laser absorption spectroscopy in the 13 560-14 050 cm(-1) spectral region. Among 437 lines attributed to HDO, 399 were assigned to the 4nu(3) highly excited overtone transition. One hundred twenty-nine experimental energy levels were derived from the spectrum identification with rotational quantum numbers J as high as 16 and K(a) as high as 7. The (004) vibrational state of HDO was found to be nearly isolated. Rotational and centrifugal distortion parameters of the effective rotational Hamiltonian in the Pade-Borel approximants form, retrieved from the fitting, allow the reproduction of the experimental energy levels with the root-mean-square deviation of 0.012 cm(-1), close to the experimental accuracy. Some rotational energy levels of the (004) state seem to be slightly perturbed by local resonances with the (052) highly excited bending state. The resonance mixing was found to be large enough to give rise to seven 5nu(2) + 2nu(3) transitions, but otherwise too weak to be observable. The maximum difference between the derived experimental energy levels and the recent high accuracy ab initio predictions (H. Partridge and D. W. Schwenke, J. Chem. Phys. 106, 4618-4639 (1997)) is -2.7 cm(-1). Copyright 1999 Academic Press.