Vibrations in the B4 rhombic structure

A double minimum six-dimensional potential energy surface (PES) is determined in symmetry coordinates for the most stable rhombic (D2h) B4 isomer in its 1Ag electronic ground state by fitting to energies calculated ab initio. The PES exhibits a barrier to the D4h square structure of 255 cm(-1). The vibrational levels (J=0) are calculated variationally using an approach which involves the Watson kinetic energy operator expressed in normal coordinates. The pattern of about 65 vibrational levels up to 1600 cm(-1) for all stable isotopomers is analyzed. Analogous to the inversion in ammonia-like molecules, the rhombus rearrangements lead to splittings of the vibrational levels. In B4 it is the B1g (D4h) mode which distorts the square molecule to its planar rhombic form. The anharmonic fundamental vibrational transitions of 11B4 are calculated to be (splittings in parentheses): G(0)=2352(22) cm(-1), nu1(A1g)=1136(24) cm(-1), nu2(B1g)=209(144) cm(-1), nu3(B2g)=1198(19) cm(-1), nu4(B2u)=271(24) cm(-1), and nu5(Eu)=1030(166) cm(-1) (D4h notation). Their variations in all stable isotopomers were investigated. Due to the presence of strong anharmonic resonances between the B1g in-plane distortion and the B2u out-of-plane bending modes, the higher overtones and combination levels are difficult to assign unequivocally.     

Observation of the d3Pi(g)<--c3Sigma(u)+ band system of C2

A new band system of C(2), d (3)Pi(g)<--c (3)Sigma(u) (+) is observed by laser induced fluorescence spectroscopy, constituting the first direct detection of the c (3)Sigma(u) (+) state of C(2). Observations were made by laser excitation of c (3)Sigma(u) (+)(v(")=0) C(2), produced in an acetylene discharge, to the d (3)Pi(g)(v(')=3) level, followed by detection of Swan band fluorescence. Rotational analysis of this band yielded rotational constants for the c (3)Sigma(u) (+)(v(")=0) state: B(0)=1.9218(2) cm(-1), lambda(0)=-0.335(4) cm(-1) and gamma(0)=0.011(2) cm(-1). The vibrational band origin was determined to be nu(3-0)=15861.28 cm(-1).     

State-to-state reaction dynamics of CH3I photodissociation at 304 nm

The detailed reaction dynamics of CH(3)I photodissociation at 304 nm were studied by using high-resolution long time-delayed core-sampling photofragment translation spectroscopy. The vibrational state distributions of the photofragment, i.e., CH(3), are directly resolved due to the high kinetic resolution of this experiment for the first time. CH(3) radicals produced from I((3)Q(0+)), I((1)Q(1) <--( 3)Q(0+)), and I((3)Q(1)) channels are populated in different vibrational state distributions. The I((3)Q(0+)) and I((3)Q(1)) channels show only progressions in the nu2'(a2") umbrella bending mode, and the I((1)Q(1) <-- (3)Q(0+)) channel shows both progression in the nu2' umbrella bending mode and a small amount of excitation in the nu1'(a1') C-H stretching mode. The photodissociation processes from the vibrational hot band of CH(3)I (upsilon3 = 1, upsilon3 = 2) were also detected, primarily because of the absorption probability from the vibrational excited states, i.e., hot bands are relatively enhanced. Photofragments from the hot bands of CH(3)I show a cold vibrational distribution compared to that from the vibrational ground state of CH(3)I. The I* quantum yield and the curve crossing possibility were also studied for the ground vibrational state of CH(3)I. The potential energy at the curve crossing point was calculated to be 32 790 cm(-1) by using the one-dimensional Landau-Zener model.     

Darling-Dennison resonance and Coriolis coupling in the bending overtones of the A 1A(u) state of acetylene, C2H2

Rotational analyses have been carried out for the overtones of the nu(4) (torsion) and nu(6) (in-plane cis-bend) vibrations of the A (1)A(u) state of C(2)H(2). The v(4)+v(6)=2 vibrational polyad was observed in high-sensitivity one-photon laser-induced fluorescence spectra and the v(4)+v(6)=3 polyad was observed in IR-UV double resonance spectra via the ground state nu(3) (Sigma(+) (u)) and nu(3)+nu(4) (Pi(u)) vibrational levels. The structures of these polyads are dominated by the effects of vibrational angular momentum: Vibrational levels of different symmetry interact via strong a-and b-axis Coriolis coupling, while levels of the same symmetry interact via Darling-Dennison resonance, where the interaction parameter has the exceptionally large value K(4466)=-51.68 cm(-1). The K-structures of the polyads bear almost no resemblance to the normal asymmetric top patterns, and many local avoided crossings occur between close-lying levels with nominal K-values differing by one or more units. Least squares analysis shows that the coupling parameters change only slightly with vibrational excitation, which has allowed successful predictions of the structures of the higher polyads: A number of weak bands from the v(4)+v(6)=4 and 5 polyads have been identified unambiguously. The state discovered by Scherer et al. [J. Chem. Phys. 85, 6315 (1986)], which appears to interact with the K=1 levels of the 3(3) vibrational state at low J, is identified as the second highest of the five K=1 members of the v(4)+v(6)=4 polyad. After allowing for the Darling-Dennison resonance, the zero-order bending structure can be represented by omega(4)=764.71, omega(6)=772.50, x(44)=0.19, x(66)=-4.23, and x(46)=11.39 cm(-1). The parameters x(46) and K(4466) are both sums of contributions from the vibrational angular momentum and from the anharmonic force field. For x(46) these contributions are 14.12 and -2.73 cm(-1), respectively, while the corresponding values for K(4466) are -28.24 and -23.44 cm(-1). It is remarkable how severely the coupling of nu(4) and nu(6) distorts the overtone polyads, and also how in this case the effects of vibrational angular momentum outweigh those of anharmonicity in causing the distortion.     

In search of definitive signatures of the elusive NCCO radical

Previous experimental assignments of the fundamental vibrational frequencies of NCCO have been brought into question by subsequent unsuccessful attempts to observe IR signatures of this radical at these frequencies. Here we compute the fundamental vibrational frequencies by applying second-order vibrational perturbation theory to the complete quartic force field computed at the all-electron (AE) coupled cluster singles, doubles, and perturbative triples level [CCSD(T)] with the correlation-consistent, polarized core-valence quadruple-zeta (cc-pCVQZ) basis set, which has tight functions to correctly describe core correlation. The AE-CCSD(T)/cc-pCVQZ geometric parameters are r(e)(N-C)=1.1623 A, r(e)(C-C)=1.4370 A, r(e)(C-O)=1.1758 A, theta(e)(N-C-C)=168.55 degrees , and theta(e)(C-C-O)=132.22 degrees . Our CCSD(T)/cc-pCVQZ values of the characteristic stretching frequencies nu(1) and nu(2) are 2171 and 1898 cm(-1), respectively, in stark contrast to the experimentally derived values of 2093 and 1774 cm(-1). Finally, focal-point extrapolations using correlation-consistent basis sets cc-pVXZ (X=D,T,Q,5,6) and electron correlation treatments as extensive as full coupled cluster singles, doubles, and triples (CCSDT) with perturbative accounting of quadruple excitations [CCSDT(Q)] determine the vibrationless barrier to linearity of NCCO and the dissociation energy (D(0)) of NCCO-->NC+CO to be 8.4 and 26.5 kcal mol(-1), respectively. Using our precisely determined dissociation energy, we recommend a new 0 K enthalpy of formation for NCCO of 50.9+/-0.3 kcal mol(-1).     

Mode selective tunneling dynamics observed by high resolution spectroscopy of the bending fundamentals of 14NH2D and 14ND2H

High resolution (0.004 and 0.01 cm(-1) instrumental bandwidth) interferometric Fourier transform infrared spectra of (14)NH2D and (14)ND2H were measured on a Bomem DA002 spectrometer in a supersonic jet expansion and at room temperature. We report the analysis of the bending fundamentals of (14)NH2D with term values Tv(s)=1389.9063(2) cm(-1) and Tv(a)=1390.4953(2) cm(-1) for the nu(4b) fundamental and Tv(s)=1605.6404(7) cm(-1) and Tv(a)=1591.0019(7) cm(-1) for the nu(4a) fundamental, and of (14)ND2H with term values of Tv(s)=1233.3740(2) cm(-1) and Tv(a)=1235.8904(2) cm(-1) for the nu(4a) fundamental and Tv(s)=1461.7941(9) cm(-1) and Tv(a)=1461.9918(19) cm(-1) for the nu(4b) fundamental. In all cases Tv(s) gives the position of the symmetric inversion sublevel (with positive parity) and Tv(a) the position of the antisymmetric inversion sublevel (with negative parity). The notation for the fundamentals nu(4a) and nu(4b) is chosen by correlation with the degenerate nu(4) mode in the C(3v) symmetric molecules NH3 and ND3. The degeneracy is lifted in Cs symmetry and a indicates the symmetric, b the antisymmetric normal mode with respect to the Cs symmetry plane in NH2D and ND2H. Assignments were established with certainty by means of ground state combination differences. About 20 molecular parameters of the effective S-reduced Hamiltonian could be determined accurately for each fundamental. In particular, the effect of Fermi resonances of the 2nu(2) overtone with the nu(4a) bending mode was observed, leading to an increased inversion splitting in the case of ND2H and to a strongly increased inversion splitting and an inverted order of the two inversion levels in NH2D. Rotational perturbations observed with the nu(4b) bending fundamentals are probably due to Coriolis interactions with the inversion overtone 2nu(2). The results are important for understanding isotope effects on the inversion in ammonia as well as its selective catalysis and inhibition by excitation of different vibrational modes, as treated by quantum dynamics on high dimensional potential hypersurfaces of this molecule.     

Rovibronic bands of the A<--X transition of CH3OO and CD3OO detected with cavity ringdown absorption near 1.2-1.4 microm

We have recorded several rovibronic bands of CH3OO and CD3OO in their A<--X transitions in the range of 1.18-1.40 microm with the cavity ringdown technique. While the electronic origins for these species have been reported previously, many newly observed rovibronic bands are described here. The experimental vibrational frequencies (given as nu in the unit cm(-1) in this paper) for the COO bending (nu8) and COO symmetric stretching (nu7) modes in the A state are 378 and 887 cm(-1) for CH3OO, and 348 and 824 cm(-1) for CD3OO, respectively. In addition, two other vibrational frequencies were observed for the A state of CD3OO, namely, nu5 (954 cm(-1)) and nu6 (971 cm(-1)). These experimental vibrational frequencies for the A state of both CH3OO and CD3OO are in good agreement with predictions from quantum-chemical calculations at the UB3LYP/aug-cc-pVTZ level. The enhanced activity of the nu5 vibrational mode in CD3OO is rationalized by mode mixing with the nu7 mode, as supported by calculations of multidimensional Franck-Condon factors. In addition, many hot bands involving the methyl torsional mode (nu12) are observed for both normal and deuterated methyl peroxy. These bands include the "typical" sequence transitions and some "atypical" ones due to the nature of the eigenvalues and eigenfunctions which are a consequence of the low, but very different, torsional barriers in the X and A states. In addition, the 12(2)2 band in CH3OO and the 12(3)3 band in CD3OO show quite different structures than the origin bands, an effect which results from tunneling splittings comparable to the rotational contour.     

Spectroscopy of the OC-HF hydrogen-bonded complex at vHF=3

The v(HF)=3 levels of the linear OC-HF complex are observed in the range of 10,800-11,500 cm(-1) using intracavity Ti-sapphire laser-induced fluorescence. The vibrational predissociation linewidths of both (30000) and (3001(1)0) states exceed 5 GHz; thus, the measured spectra are not rotationally resolvable. Under the assumption that these levels are not strongly perturbed, the rotational constants of the two levels are determined to be 0.1100(1) cm(-1) for (30000), 0.1081(1), and 0.1065(1) cm(-1) for f and e sublevels of (3001(1)0), respectively, through band contour fitting. The (30000)<--(00000) band origin is at 10,894.46(1) cm(-1), showing a HF wave number redshift of 478.3 cm(-1). The 4.07 redshift ratio of v(HF)=3 to that of v(HF)=1 indicates a significantly nonlinear increase of the intermolecular interaction energy through HF valence excitation. An ab initio interaction potential surface for HF valence coordinates varying from 0.8 to 1.25 A is used to examine vibrational dynamics. The HF valence vibration v(1) is treated perturbatively, showing that the vibrational redshifts are determined essentially in first order with only a very small second-order contribution. The (3001(1)0)<--(00000) combination transition is observed with the band origin at 11,432.66(1) cm(-1), giving the HF intermolecular bending mode to be 538.2 cm(-1). The high frequency of this vibration, compared to that in similar HF complexes, shows the strong angular anisotropy of the intermolecular interaction potential of OC-HF with respect to the HF subunit. The lifetime of the (3001(1)0) level increases to 28 ps from 14 ps for (30000).     

Electronic and vibrational spectroscopy and vibrationally mediated photodissociation of V+(OCO)

Electronic spectra of gas-phase V+(OCO) are measured in the near-infrared from 6050 to 7420 cm(-1) and in the visible from 15,500 to 16,560 cm(-1), using photofragment spectroscopy. The near-IR band is complex, with a 107 cm(-1) progression in the metal-ligand stretch. The visible band shows clearly resolved vibrational progressions in the metal-ligand stretch and rock, and in the OCO bend, as observed by Brucat and co-workers. A vibrational hot band gives the metal-ligand stretch frequency in the ground electronic state nu3' = 210 cm(-1). The OCO antisymmetric stretch frequency in the ground electronic state (nu1') is measured by using vibrationally mediated photodissociation. An IR laser vibrationally excites ions to nu1' = 1. Vibrationally excited ions selectively dissociate following absorption of a second, visible photon at the nu1' = 1 <-- nu1' = 1 transition. Rotational structure in the resulting vibrational action spectrum confirms that V+(OCO) is linear and gives nu1' = 2392.0 cm(-1). The OCO antisymmetric stretch frequency in the excited electronic state is nu1' = 2368 cm(-1). Both show a blue shift from the value in free CO2, due to interaction with the metal. Larger blue shifts observed for complexes with fewer ligands agree with trends seen for larger V+(OCO)n clusters.     

Assignment of rovibrational transitions of propyne in the region of 2934-2952 cm(-1) measured by two-color IR-vacuum ultraviolet laser photoion-photoelectron methods

The infrared (IR) spectrum of propyne in the region of 2934-2952 cm(-1) has been recorded by the IR-vacuum ultraviolet (VUV)-photoion method. The spectrum is shown to consist of two near-resonant, but noncoupled vibrational bands: the nu2 symmetric methyl C-H stretching vibrational band and a combination vibrational band nucs. The previously unobserved Q line of the nucs band is observed. The rotational transition lines of the nu2=1 band produces IR-VUV-pulsed field ionization-photoelectron (IR-VUV-PFI-PE) signal at the C3H4+ (nu2+=1) photoionization threshold. The rotational transition lines associated with the nucs band do not produce IR-VUV-PFI-PE signal. Rotational transition lines of both vibrational bands are assigned and simulated; and ab initio calculations further confirm the assignment.     

Jet cooled spectroscopy of H2DO+: Barrier heights and isotope-dependent tunneling dynamics from H3O+ to D3O+

The first high resolution spectroscopic data for jet cooled H2DO+ are reported, specifically via infrared laser direct absorption in the OH stretching region with a slit supersonic jet discharge source. Transitions sampling upper (0-) and lower (0+) tunneling states for both symmetric (nu1+ <-- 0+, nu1- <-- 0-, and nu1- <-- 0+) and antisymmetric (nu3+ <-- 0+ and nu3- <-- 0-) OH stretching bands are observed, where +/- refers to wave function reflection symmetry with respect to the planar umbrella mode transition state. The spectra can be well fitted to a Watson asymmetric top Hamiltonian, revealing band origins and rotational constants for benchmark comparison with high-level ab initio theory. Of particular importance are detection and assignment of the relatively weak band (nu1- <-- 0+) that crosses the inversion tunneling gap, which is optically forbidden in H3O+ or D3O+, but weakly allowed in H2DO+ by lowering of the tunneling transition state symmetry from D(3h) to C(2v). In conjunction with other H2DO+ bands, this permits determination of the tunneling splittings to within spectroscopic precision for each of the ground [40.518(10) cm(-1)], nu1 = 1 [32.666(6) cm(-1)], and nu3 = 1 [25.399(11) cm(-1)] states. A one-dimensional zero-point energy corrected potential along the tunneling coordinate is constructed from high-level ab initio CCSD(T) calculations (AVnZ, n = 3,4,5) and extrapolated to the complete basis set limit to extract tunneling splittings via a vibrationally adiabatic treatment. Perturbative scaling of the potential to match splittings for all four isotopomers permits an experimental estimate of DeltaV0 = 652.9(6) cm(-1) for the tunneling barrier, in good agreement with full six-dimensional ab initio results of Rajamaki, Miani, and Halonen (RMH) [J. Chem. Phys. 118, 10929 (2003)]. (DeltaV0 (RMH) = 650 cm(-1)). The 30%-50% decrease in tunneling splitting observed upon nu1 and nu3 vibrational excitations arises from an increase in OH stretch frequencies at the planar transition state, highlighting the transition between sp2 and sp3 hybridizations of the OHD bonds as a function of inversion bending angle.     

Vibrational overtone spectroscopy of jet-cooled methanol from 5000 to 14 000 cm(-1)

Spectra of jet-cooled methanol in the overtone and combination region from 5000 to 14 000 cm(-1) have been obtained by means of infrared laser-assisted photofragment spectroscopy. Many of the observed features are assigned to combination bands of the type nnu(1)+nu(6), nnu(1)+nu(8), and nnu(1)+nu(6)+nu(8) (n=1,2,3), where nu(1) is the OH stretch, nu(6) is the OH bend, and nu(8) is the CO stretch. These bands show sharp torsion-rotation structure with features as narrow as 0.1 cm(-1). We also observe CH stretch overtones that are weaker than the OH containing combination bands and lack distinct torsion-rotation structure above v(CH)=2. The extent of observed structure on these bands allows us to place limits on the intramolecular vibrational energy redistribution decay rates in the upper vibrational states. We report a global fit of the observed band centers to a simple expression involving low-order anharmonicity constants.     

Infrared spectra and ab initio calculations for the F- -(CH4)n (n = 1-8) anion clusters

Infrared spectra of mass-selected F- -(CH4)n (n = 1-8) clusters are recorded in the CH stretching region (2500-3100 cm-1). Spectra for the n = 1-3 clusters are interpreted with the aid of ab initio calculations at the MP2/6-311++G(2df 2p) level, which suggest that the CH4 ligands bind to F- by equivalent, linear hydrogen bonds. Anharmonic frequencies for CH4 and F--CH4 are determined using the vibrational self-consistent field method with second-order perturbation theory correction. The n = 1 complex is predicted to have a C3v structure with a single CH group hydrogen bonded to F-. Its spectrum exhibits a parallel band associated with a stretching vibration of the hydrogen-bonded CH group that is red-shifted by 380 cm-1 from the nu1 band of free CH4 and a perpendicular band associated with the asymmetric stretching motion of the nonbonded CH groups, slightly red-shifted from the nu3 band of free CH4. As n increases, additional vibrational bands appear as a result of Fermi resonances between the hydrogen-bonded CH stretching vibrational mode and the 2nu4 overtone and nu2+nu4 combination levels of the methane solvent molecules. For clusters with n < or = 8, it appears that the CH4 molecules are accommodated in the first solvation shell, each being attached to the F- anion by equivalent hydrogen bonds.     

Vibrational mode frequencies of silica species in SiO2-H2O liquids and glasses from ab initio molecular dynamics

Vibrational spectroscopy techniques are commonly used to probe the atomic-scale structure of silica species in aqueous solution and hydrous silica glasses. However, unequivocal assignment of individual spectroscopic features to specific vibrational modes is challenging. In this contribution, we establish a connection between experimentally observed vibrational bands and ab initio molecular dynamics (MD) of silica species in solution and in hydrous silica glass. Using the mode-projection approach, we decompose the vibrations of silica species into subspectra resulting from several fundamental structural subunits: The SiO(4) tetrahedron of symmetry T(d), the bridging oxygen (BO) Si-O-Si of symmetry C(2v), the geminal oxygen O-Si-O of symmetry C(2v), the individual Si-OH stretching, and the specific ethane-like symmetric stretching contribution of the H(6)Si(2)O(7) dimer. This allows us to study relevant vibrations of these subunits in any degree of polymerization, from the Q(0) monomer up to the fully polymerized Q(4) tetrahedra. Demonstrating the potential of this approach for supplementing the interpretation of experimental spectra, we compare the calculated frequencies to those extracted from experimental Raman spectra of hydrous silica glasses and silica species in aqueous solution. We discuss observed features such as the double-peaked contribution of the Q(2) tetrahedral symmetric stretch, the individual Si-OH stretching vibrations, the origin of the experimentally observed band at 970 cm(-1) and the ethane-like vibrational contribution of the H(6)Si(2)O(7) dimer at 870 cm(-1).     

Rovibrationally selected and resolved state-to-state photoionization of ethylene using the infrared-vacuum ultraviolet pulsed field ionization-photoelectron method

By preparing ethylene [C2H4(X1Ag)] in selected rotational levels of the nu11(b1u), nu2+nu12(b1u), or nu9(b2u) vibrational state with infrared (IR) laser photoexcitation prior to vacuum ultraviolet (VUV) laser photoionization, we have recorded rotationally resolved pulsed field ionization-photoelectron (PFI-PE) spectra for C2H4+(X2B3u) in the energy region of 0-3000 cm(-1) above the ionization energy (IE) of C2H4(X1Ag). Here, nu2(ag), nu9(b2u), nu11(b1u), and nu12(b1u) represent the C-C stretching, CH2 stretching, CH2 stretching, and CH2 bending modes of C2H4(X1Ag), respectively. The fully rovibrationally resolved spectra have allowed unambiguous symmetry assignments of the observed vibrational bands, which in turn have provided valuable information on the photoionization dynamics of C2H4. The IR-VUV photoionization of C2H4(X1Ag) via the nu11(b1u) or nu2+nu12(b1u) vibrational states is found to predominantly produce vibrational states of C2H4+(X2B3u) with b1u symmetry, which cannot be observed in single-photon VUV-PFI-PE measurements of C2H4(X1Ag). The analysis of the observed IR-VUV-PFI-PE bands has provided the IE(C2H4) = 84,790.2(2) cm(-1) and accurate vibrational frequencies for the nu4+(au)[84.1(2) cm(-1)], nu12+(b1u)[1411.7(2) cm(-1)], nu4+ +nu12+(b1g)[1482.5(2) cm(-1)], nu2+(ag)[1488.3(2) cm(-1)], nu2+ + nu4+(au)[1559.2(2) cm(-1)], 2nu4+ + nu12 +(b1u)[1848.5(2) cm(-1)], 4nu4+ + nu12 +(b1u)[2558.8(2) cm(-1)], nu2+ + nu12 +(b1u)[2872.7(2) cm(-1)], and nu11+(b1u)[2978.7(2) cm(-1)] vibrational states of C2H4+(X2B3u), where nu4+ is the ion torsional state. The IE(C2H4) and the nu4+(au), nu2+(ag), and nu2+ + nu4+ (au) frequencies are in excellent accord with those obtained in previous single-photon VUV-PFI-PE measurements. The other ion vibrational frequencies represent new experimental determinations. We have also performed high-level ab initio anharmonic vibrational frequency calculations for C2H4(X1Ag) and C2H4+(X2B3u) at the CCSD(T)/aug-cc-pVQZ level for guidance in the assignment of the IR-VUV-PFI-PE spectra. All theoretical vibrational frequencies for the neutral and ion, except the ion torsional frequency, are found to agree with experimental vibrational frequencies to better than 1%.     

The near infrared spectrum of ozone by CW-cavity ring down spectroscopy between 5850 and 7000 cm(-1): new observations and exhaustive review

Weak vibrational bands of (16)O(3) could be detected in the 5850-7030 cm(-1) spectral region by CW-cavity ring down spectroscopy using a set of fibered DFB diode lasers. As a result of the high sensitivity (noise equivalent absorption alpha(min) approximately 3 x 10(-10) cm(-1)), bands reaching a total of 16 upper vibrational states have been previously reported in selected spectral regions. In the present report, the analysis of the whole investigated region is completed by new recordings in three spectral regions which have allowed: (i) a refined analysis of the nu(1) + 3nu(2) + 3nu(3) band from new spectra in the 5850-5900 cm(-1) region; (ii) an important extension of the assignments of the 2nu(1)+5nu(3) and 4nu(1) + 2nu(2) + nu(3) bands in the 6500-6600 cm(-1) region, previously recorded by frequency modulation diode laser spectroscopy. The rovibrational assignments of the weak 4nu(1) + 2nu(2) + nu(3) band were fully confirmed by the new observation of the 4nu(1) + 2nu(2) + nu(3)- nu(2) hot band near 5866.9 cm(-1) reaching the same upper state; (iii) the observation and modelling of three A-type bands at 6895.51, 6981.87 and 6990.07 cm(-1) corresponding to the highest excited vibrational bands of ozone detected so far at high resolution. The upper vibrational states were assigned by comparison of their energy values with calculated values obtained from the ground state potential energy surface of (16)O(3). The vibrational mixing and consequently the ambiguities in the vibrational labelling are discussed. For each band or set of interacting bands, the spectroscopic parameters were determined from a fit of the corresponding line positions in the frame of the effective Hamiltonian (EH) model. A set of selected absolute line intensities was measured and used to derive the parameters of the effective transition moment operator. The exhaustive review of the previous observations gathered with the present results is presented and discussed. It leads to a total number of 3863 energy levels belonging to 21 vibrational states and corresponding to 7315 transitions. In the considered spectral region corresponding to up to 82% of the dissociation energy, the increasing importance of the "dark" states is illustrated by the occurrence of frequent rovibrational perturbations and the observation of many weak lines still unassigned.     

Fermi interaction between the nu1 and the nu2+4 nu(s) bands of Ar...DN2(+)

Two rotationally fully resolved vibrational bands have been assigned unambiguously to the linear deuteron bound Ar...DN(2) (+) complex by using ground state combination differences. The ionic complex is formed in a supersonic planar plasma expansion optimized and controlled by a mass spectrometer and is detected in direct absorption using tunable diode lasers and applying production modulation spectroscopy. The band origins are located at 2436.272 cm(-1) and at 2435.932 cm(-1) and correspond to the nu(1) band (NN stretch) and to the nu(2)+4 nu(s) combination band (DN and intermolecular stretch), respectively. The two bands overlap strongly and the large intensity of the combination band is explained in terms of a Fermi interaction. This interaction perturbs the observed transitions, particularly for low J values. Least-squares fitting yields values for the Fermi interaction parameters of F(0)=0.332 cm(-1) and F(J)=-0.001 46 cm(-1) and results in accurate rotational constants. These are discussed both from an experimental and a theoretical point of view.     

Origin of methyl torsional barrier in 1-methyl-2-(1H)-pyridone

The laser induced fluorescence excitation and single vibronic excitation dispersed fluorescence spectra have been studied for supersonic jet cooled 1-methyl-2(1h)-pyridone. The methyl torsional bands and some low frequency vibrational transitions were assigned for both ground and excited states. The torsional parameters V(3)=244 cm(-1) and V(6)=15 cm(-1) for the ground state and V(3)=164 cm(-1) and V(6)=40 cm(-1) for the excited state were obtained. To get the insight into the methyl torsional barrier, ab initio calculations were performed and compared with the experimental results. Origin of potential barrier was traced by partitioning the barrier energy into changes in bond-antibond interaction, structural, and steric energies accompanying methyl rotation using natural bond orbital analysis. The role of local interactions in ascertaining the barrier potential reveals that its nature cannot be understood without considering the molecular flexing. The hyperconjugation between CHsigma(*) and ring pi(*) observed in lowest unoccupied molecular orbital (LUMO) stabilizes the methyl group conformer that undergoes a 60 degrees rotation in the excited state with respect to that of the ground state, and it is the change in LUMO that plays important role in the excited state barrier formation.     

High-resolution infrared spectroscopy in the 1,200-1,300 cm(-1) region and accurate theoretical estimates for the structure and ring-puckering barrier of perfluorocyclobutane

We present experimental infrared spectra and theoretical electronic structure results for the geometry, anharmonic vibrational frequencies, and accurate estimates of the magnitude and the origin of the ring-puckering barrier in C4F8. High-resolution (0.0015 cm-1) spectra of the nu12 and nu13 parallel bands of perfluorocyclobutane (c-C4F8) were recorded for the first time by expanding a 10% c-C4F8 in helium mixture in a supersonic jet. Both bands are observed to be rotationally resolved in a jet with a rotational temperature of 15 K. The nu12 mode has b2 symmetry under D2d that correlates to a2u symmetry under D4h and consequently has +/- <-- +/- ring-puckering selection rules. A rigid rotor fit of the nu12 band yields the origin at 1292.56031(2) cm-1 with B' = 0.0354137(3) cm-1 and B' ' = 0.0354363(3) cm-1. The nu13 mode is of b2 symmetry under D2d that correlates to b2g under D4h, and in this case, the ring-puckering selection rules are +/- <-- -/+ . Rotational transitions from the ground and first excited torsional states will be separated by the torsional splitting in the ground and excited vibrational states, and indeed, we observe a splitting of each transition into strong and weak intensity components with a separation of approximately 0.0018 cm-1. The strong and weak sets of transitions were fit separately again using a rigid rotor model to give nu13(strong) = 1240.34858(4) cm-1, B' = 0.0354192(7) cm-1, and B' ' = 0.0354355(7) cm-1 and nu13(weak) = 1240.34674(5) cm-1, B' = 0.0354188(9) cm-1, and B' ' = 0.0354360(7) cm-1. High-level electronic structure calculations at the MP2 and CCSD(T) levels of theory with the family of correlation consistent basis sets of quadruple-zeta quality, developed by Dunning and co-workers, yield best estimates for the vibrationally averaged structural parameters r(C-C) = 1.568 A, r(C-F)alpha = 1.340 A, r(C-F)beta = 1.329 A, alpha(F-C-F) = 110.3 degrees , thetaz(C-C-C) = 89.1 degrees , and delta(C-C-C-C) = 14.6 degrees and rotational constants of A = B = 0.03543 cm-1 and C = 0.02898 cm-1, the latter within 0.00002 cm-1 from the experimentally determined values. Anharmonic vibrational frequencies computed using higher energy derivatives at the MP2 level of theory are all within <27 cm-1 (in most cases <5 cm-1) from the experimentally measured fundamentals. Our best estimate for the ring-puckering barrier at the CCSD(T)/CBS (complete basis set) limit is 132 cm-1. Analysis of the C4F8 electron density suggests that the puckering barrier arises principally from the sigmaCC-->sigmaCF hyperconjugative interactions that are more strongly stabilizing in the puckered than in the planar form. These interactions are, however, somewhat weaker in C4F8 than in C4H8, a fact that is consistent with the smaller barrier in the former (132 cm-1) with respect to the latter (498 cm-1).