Electric-resonance Optothermal Spectrometer Research Articles

Applications of Fourier transform microwave (FTMW) detected infrared–microwave double-resonance spectroscopy to problems in vibrational dynamics

A description of Fourier transform microwave (FTMW) detected infrared (IR) spectroscopy is presented. A series of measurements demonstrates the versatility of using a narrowband high Q cavity FTMW spectrometer as a rotationally resolved detector for molecular-beam infrared spectroscopy. The IR-FTMW spectrometer performance is characterized by comparing the spectrum of the acetylenic C–H stretch fundamental of 1-butyne to that obtained using a high-resolution electric resonance optothermal spectrometer (EROS). Two different measurement schemes, corresponding to IR excitation before or after the microwave polarization pulse, are compared. The ability to measure rotationally resolved spectra in the region of the first overtone of the acetylenic C–H stretch is demonstrated through measurements on tertbutylacetylene and trimethylsilylacetylene. One major advantage of the current technique is that large frequency scan rates can be achieved. This feature is useful for molecules exhibiting fast intramolecular vibrational energy redistribution (IVR) as shown by measurements of the highly perturbed acetylenic C–H stretch fundamental region of cyclopropylacetylene. The second strength of the technique is that the high-sensitivity of FTMW spectroscopy makes it possible to obtain the infrared spectra of molecules in low abundance in the pulsed jet expansion. This capability is exploited to measure the infrared spectrum of the 13C isotopomer of cyclopropylacetylene in natural abundance and to obtain the spectra of the trans and gauche conformational isomers of 4-fluorobut-1-yne. The technique is also well-suited to measuring the vibrational spectra of weakly bound complexes. The spectral simplification achieved by the double-resonance measurement has made it possible to identify weak perturbations in the acetylenic C–H stretch fundamental region of the H–C C–H–NH 3 complex. The acetylenic C–H stretch fundamental spectrum of propyne–ammonia has also been obtained using the technique.

Rotation-tunneling spectrum of the deuterated ammonia dimer

The millimeter and submillimeter-wave molecular-beam spectrum of the perdeuterated ammonia dimer (ND3)2 has been measured between approximately 50 and 400 GHz using an electric-resonance optothermal spectrometer (EROS). As in the case of the (NH3)2, the spectrum is complicated by the threefold internal rotation of the ND3 subunits, the interchange tunneling of the two subunits, and the inversion of the subunits through their respective centers of masses. These tunneling motions split the rigid-molecule energy levels into 22 components, which all have nonzero statistical weights in the case of the deuterated dimer. Transitions have been assigned for rotation-tunneling states correlating to A–A (ortho–ortho) combinations of the ND3 monomer states, where A designates the rovibronic symmetries of the ND3 subunits. One K=1←1, one K=1←0, one K=0←1, and two K=0←0 progressions have been assigned. The data have been fit to 0.28 MHz using linear molecule-type energy-level expressions to determine rotational constants, band origins, l/K-type double constants, and centrifugal distortion constants. The two K=0←0 subbands, with origins near 264 GHz, are split by 64 MHz due to monomer inversion, as observed previously in the NH3 dimer. The 264 GHz, K=0 splitting arises predominantly from monomer interchange tunneling and is nearly a factor of 2 less than the 483 GHz value for the NH3 dimer. The separation is also approximately 25% smaller than predicted by Olthof et al. [E. H. T. Olthof, A. van der Avoird, and P. E. S. Wormer, J. Chem. Phys. 101, 8430 (1994)] from dynamical calculations on a model potential energy surface adjusted to fit the observed far-infrared rotation-tunneling spectrum of the NH3 dimer.

Intramolecular Vibrational Dynamics of the Asymmetric [dbnd]CH2Hydride Stretch of Isobutene

The eigenstate-resolved, high-resolution (5 MHz) infrared spectrum of the asymmetric CH2hydride stretch of isobutene has been measured using an electric resonance optothermal spectrometer (EROS). This vibrational band near 3087 cm−1was rotationally assigned with ground state microwave–infrared double-resonance spectroscopy. IVR rates from rotationally homogeneous IVR multiplets at values of total angular momentum,Jranging fromJ= 0 toJ= 3 andKavalues ofKa= 0 toKa= 2, were obtained. The average IVR lifetime for this vibrational mode is 105 ps and independent of rotational state. The experimental state density of the rotationless 000vibrational state, approximately 150 states/cm−1, is in good agreement with the direct count result of 99 states/cm−1using theC+3v⊗C−3v(G36) molecular symmetry group. The lineshape profiles of the IVR multiplets are investigated in order to elucidate information concerning the dependence of IVR rates on the symmetry of the torsional wavefunction. We find that there is a common IVR rate among the various torsional symmetries. IVR lifetimes observed in the present study are compared to other asymmetric ethylenic hydride stretch IVR rates measured in our laboratory.

Molecular-Beam Electric-Resonance Optothermal Spectroscopy Study of the Rotational Spectrum of the Less Stable Conformer of Methyl Vinyl Ether

The pure rotational spectrum of the less stable conformer of methyl vinyl ether has been measured in an electric-resonance optothermal spectrometer. Analysis of the microwave spectrum shows that the less stable conformer is effectively planar with a low barrier between twogaucheforms of the conformer. The rotational constants for the high-energy conformer, obtained from a least-squares fit to a threefold barrier internal rotation Hamiltonian areA= 39 192.490(26) MHz,B= 4527.5496(39) MHz, andC= 4181.2146(43) MHz. The threefold barrier to internal rotation is determined to beV3= 427.516(71) cm−1. The dipole moment for the conformer has been measured using the Autler–Townes splitting of states in a strong, resonant microwave field. The dipole moment is μtot= 1.9(2) D [μa= 1.3(1) D, μb= 1.4(1) D]. The effectively planar structure of the conformer and the magnitude of the dipole moment are in good agreement with previousab initiocalculations. The experimentally determined threefold barrier to methyl rotation of the present study also agrees well with the value calculated fromab initiomethods, 515 cm−1.

High-Resolution Microwave and Infrared Molecular-Beam Studies of the Conformers of 1,1,2,2-Tetrafluoroethane

High-resolution microwave and infrared molecular-beam spectra have been measured for 1,1,2,2-tetrafluoroethane (HFC134). For the higher energy, polar,C2symmetry,gaucheconformer, microwave spectra have been recorded for the normal and mono-13C isotopomers and analyzed to determine a C–C bond length of 1.512(4) Å, in good agreement with a recentab initiovalue (MP2/6-31G**) of 1.515 Å [S. Papasavva, K. H. Illinger, and J. E. Kenny,J. Phys. Chem.100, 10100–10110 (1996)]. A tunable microwave-sideband CO2laser and electric-resonance optothermal spectrometer have been used to measure the infrared spectrum of the ν6, C–C stretch of thegaucheconformer near 906 cm−1. Microwave-infrared double resonance and precise ground state combination differences provided by the microwave measurements guide the assignment of the spectrum. The observation of ac-type spectrum definitively establishes that the upper state vibration is ofAsymmetry in theC2point group. The spectrum is fit to a Watson asymmetric-top Hamiltonian to a standard deviation of 0.24 MHz. A weak perturbation shifts the line positions for transitions nearJ = Kc= 20 by as much as 12 MHz. The identity of the perturber is unknown. Pulsed slit-jet diode-laser spectra have been recorded for the ν16vibration of theanticonformer near 1127 cm−1. Ana- andc-type hybrid band is observed, consistent with aBusymmetry mode. Previous low-resolution studies have attributed the 1127-cm−1mode to either aBuor anAusymmetry vibration. A total of 522 nonblended transitions were assigned and fit to determine ground and excited state constants. The ground state constants ofA= 5134.952(65) MHz,B= 3148.277(27) MHz, andC= 2067.106(43) MHz are the first experimental determinations of the rotational constants for this conformer. Here, typeAstandard uncertainties are given in the parentheses.

Conformational Dependence of Intramolecular Vibrational Energy Redistribution in the EthylenicCH2 Stretch Fundamental of Allyl Fluoride

The high-resolution (6 MHz) infrared spectra of the asymmetric ethylenic hydride stretches of both cis and gauche allyl fluoride have been measured. Rotational assignments of the eigenstates are made using the ground-state microwave-infrared double-resonance capabilities of an electric resonance optothermal spectrometer (EROS). The cis vibrational band near 3114 cm-1 is characterized by sparse, narrow IVR multiplets resulting from weak Coriolis or cross conformer interactions between the vibrational bath states. The IVR lifetime of the cis vibrational band is approximately 2 ns (2000 ps). The gauche vibrational band near 3100 cm-1 is qualitatively and quantitatively quite different from the cis band. The gauche band is characterized by significantly fragmented IVR multiplets with an average IVR lifetime of about 90 ps. The measured anharmonic state density of the gauche band is about 30 states/cm-1. The disagreement between the measured state densities of the gauche vibrational band and the calculated val...

Molecular-beam infrared–infrared double-resonance spectroscopy study of the vibrational dynamics of the acetylenic C–H stretch of propargyl amine

The acetylenic C–H stretch spectrum of propargyl amine near 3330 cm−1 has been measured at 0.0002 cm−1 (6 MHz) resolution with a tunable color-center laser in an electric-resonance optothermal spectrometer. The spectrum has been fully assigned through IR–IR double resonance measurements employing a tunable, microwave sideband-CO2 laser. The 10 μm spectrum of propargyl amine displays splittings in the two nuclear spin symmetry states arising from amino-proton interchange, allowing double-resonance assignment of the –NH2 group resultant proton nuclear spin quantum number in the highly fragmented 3 μm spectrum. The experimental state density is consistent with a (2J+1) increase that is expected if all near-resonant states are coupled. From this J-dependent growth in the state density we determine the density of states at J=0 to be 22 states/cm−1. This value is in reasonable agreement with the direct state count result of 16 states/cm−1. The unperturbed transition frequencies for the two different nuclear spin species at a given rotational level do not coincide, differing on average by about 50 MHz. The nonresonant coupling effects which produce effective splittings in the 10 μm spectrum appear to survive into the high state density regime. The measured IVR lifetimes are on the order of 500 ps for the low Ka values studied here (Ka<4) and show a Ka-dependence with the IVR rate increasing as Ka increases. The statistical properties of the spectrum have been compared to predictions from random matrix theory. The level spacings are not well represented by Wigner statistics as would be expected for underlying chaotic classical dynamics. However, the intensity fluctuations are consistent with a χ2-distribution, expected for classically chaotic systems, as measured by Heller’s F-statistic.

Doorway state enhanced intramolecular vibrational energy redistribution in the asymmetric =CH2 hydride stretch of methyl vinyl ether

The microwave-infrared double-resonance capabilities of an electric-resonance optothermal spectrometer have been used to assign the high resolution (5 MHz) infrared spectrum of the asymmetric =CH2 stretch of the cis conformer of methyl vinyl ether near 3130 cm−1. This vibrational state is anharmonically coupled to a near-resonant bath state by a 0.69 cm−1 matrix element resulting in two vibrational bands separated by about 1.44 cm−1. The two mixed states resulting from this interaction are further coupled to other near-resonant bath states with an average matrix element of about 0.01 cm−1. The coupled state density increases weakly with the total angular momentum, J, however, the intramolecular vibrational energy distribution (IVR) rate is approximately independent of the total angular momentum quantum number. Therefore, the rotationally mediated coupling mechanisms are weaker than the anharmonic terms in the redistribution process. A two-state analysis of the strong coupling, which includes a phenomenological IVR rate constant, suggests that the IVR rate in the two mixed states is dominated by the contribution from the coupled dark state. From the deconvolution of the IVR rates to remove the contribution from the dark state, the IVR lifetime of the asymmetric =CH2 stretch is determined to be 660 ps.

Rotational and Vibrational Spectroscopy and Ideal Gas Heat Capacity of HFC 134a (CF3CFH2)

The hydrofluorocarbon HFC 134a (CF3CFH2) is the primary replacement for the chlorofluorocarbon CFC 12 (CF2Cl2) in numerous applications, including automobile air conditioning and home and commercial refrigeration. Here we describe a comprehensive spectroscopic study of this molecule. Precise microwave frequencies and molecular constants have been obtained for the vibrational ground state with a pulsed-molecular-beam Fourier-transform microwave spectrometer. New isotopic ground-state microwave measurements have also been made to improve the ground-state structural determination. Infrared and Raman spectra have been obtained, and all 18 vibrations have been observed and assigned. A high-resolution (3 MHz) microwave-sideband CO2 laser and an electric-resonance optothermal spectrometer have been used to observe the molecular-beam infrared spectrum in the vicinity of the low-resolution gas-phase feature at 975 cm-1 assigned here and in some of the earlier studies as the ν15, A‘‘-symmetry, CH2 rock. Two nearly equal-intensity c-type bands are observed under high resolution with origins at 974.35 and 974.87 cm-1. The presence of two vibrational bands of A‘‘ symmetry is attributed to strong anharmonic mixing of the ν15 vibration with a nearby combination vibration. On the basis of our low-resolution infrared measurements, we identify the perturbing state as the 3ν18 + ν8 combination level. Finally, the experimental results are used to calculate the vibrational contribution to the heat capacity and are compared with the results of earlier experimental and theoretical work, including our own electronic-structure calculations using an HF/6-31G* basis set.

Microwave and Submillimeter-wave Spectra of the Mixed Deuterated–Protonated Water-Dimer Isotopomers

The microwave and submillimeter-wave rotation-tunneling spectra of several mixed deuterated–protonated isotopomers of the water dimer have been measured up to 460 GHz using an electric-resonance optothermal spectrometer, a pulsed-nozzle Fourier-transform microwave spectrometer, and a far-infrared CO2-laser difference-frequency spectrometer. Spectra were recorded for thea-typeKa= 0–0 and 1–1 andb- and/orc-typeKa= 1–0 bands for H2O—DOD, D2O—HOH, D2O—DOH, H2O—HOD, D2O—HOD, H2O—DOH, DHO—HOH, and DHO—DOD, where O—D or O—H denotes the two atoms directly involved in the hydrogen bonding. Spectra of D2O—HOH, H2O—HOD, D2O— HOD, and HDO—HOH, which are higher-energy isomers of H2O—DOD, H2O—DOH, D2O—DOH, and H2O—DOH, respectively, have not been reported previously. These higher-energy isomers are not significantly populated in Ar molecular beams, such as used in many of the previous microwave studies. Here, we obtain information on two of the three major tunneling processes found in the water dimer, the tunneling interchange of the two protons/deuterons on the proton-acceptor subunit and the tunneling interchange of the two protons/deuterons on the proton-donor subunit. The observed rotation-tunneling selection rules are consistent with previously proposed pathways for these tunneling processes. For H2O—DOD, D2O—HOH, and DHO—HOH we find donor interchange tunneling splittings of 24.4(11), 864.14(29), and 1809.39(9) MHz, respectively, averaged overKa= 0 and 1, and for DHO—DOD we find a donor-interchange tunneling splitting of 16.8(1) MHz forKa= 0. For DHO—DOD we also determined the proton/deuteron acceptor interchange tunneling splitting forKa= 0 as 107 723.7(1) MHz, similar to the 117 441.0(2) MHz valued determined previously for DHO—HOD. Because the position of the D/H involved in the hydrogen bonding is nearly coincident with the center of mass of the dimer, isotopomers which only differ by isotopic nuclei at this position have strikingly similar spectra. The present measurements furnish rotational constant and tunneling splitting data to use in the testing and refinement of proposed intermolecular potentials describing water dimer.

Vibrational couplings and energy flow in complexes of NH3 with HCN, HCCH, and HCCCCH

A tunable color-center laser and a molecular-beam electric-resonance optothermal spectrometer have been used to record the infrared spectra of the C–H stretching vibrations of HCCH–NH3, HCCD–NH3, NCH–NH3, and HCCCCH–NH3. The hydrogen-bonded C–H stretching vibrations of NCH–NH3, HCCH–NH3, and HCCCCH–NH3 are redshifted by 200.88126(30), 75.1042(38), and 127.4(1) cm−1 from the respective free monomer modes. The non-hydrogen-bonded C–H stretches are less perturbed by complexation, being blueshifted by 0.2992(3) cm−1 in HCCCCH–NH3 and redshifted by 1.179(1) cm−1 in HCCD–NH3. Consistent with the much larger perturbation of the monomer vibration for the bonded C–H stretch, the B rotational constants increase by 1%–2% for the bonded C–H stretch excited NCH–NH3 and HCCH–NH3 complexes, but change by less than 0.1% for the nonbonded C–H stretches in HCCCCH–NH3 and HCCD–NH3. The decoupling of the two C–H stretches in HCCH–NH3 is not sufficient to allow the observation of the nonbonded C–H stretch in the complex, which correlates to the Raman-active symmetric C–H stretch of acetylene. Also, no spectra were observed for the weaker N–H stretching vibrations of the complexes, consistent with the very weak intensities of these modes in the monomer. The homogeneous linewidths of the transitions, assumed to be a measure of the vibrational predissociation rate, are approximately two orders of magnitude larger for the bonded C–H stretches than for the nonbonded C–H stretches. The similarity in homogeneous widths for the nonbonded C–H stretches in HCCD–NH3 and HCCCCH–NH3, of 7–12 MHz, suggests that the rate of vibrational energy flow along acetylene chains is only weakly dependent on chain length.

The microwave spectrum and OH internal rotation dynamics of gauche-2,2,2-trifluoroethanol

The microwave spectra of CF3CH2OH and CF3CH2OD have been investigated from 5 to 26 GHz with a pulsed-nozzle Fourier-transform microwave spectrometer and from 26 to 42 GHz with an electric resonance optothermal spectrometer. Tunneling of the OH proton between the two isoenergetic gauche conformations splits the observed transitions into two tunneling components. An effective rotation-tunneling Hamiltonian is used to fit the aan both isotopomers to better than 5 and 13 kHz for the OH and OD forms, respectively. The tunneling splittings determined from the fits for the OH and OD isotopomers are 5868.6952(16) and 208.5037(42) MHz, respectively. A structural analysis using the moments of inertia of the OH and OD isotopomers determines that the hydroxyl hydrogen is directed toward the fluorine with a F...H separation of 2.561(1) Å and a dihedral angle of φ(CCOH)=68.97(6)°. The observed tunneling splittings are fit to a double-minimum potential, giving gauche–gauche tunneling barriers of 763 and 720 cm−1 and OH torsional fundamental frequencies of 364 and 271 cm−1 for CF3CH2OH and CF3CH2OD, respectively. The uncertainties shown in parentheses throughout the paper are one standard deviation.

Infrared Spectra of the 10-μm Bands of 1,2-Difluoroethane and 1,1,2-Trifluoroethane: Vibrationally Mediated Torsional Tunneling in 1,1,2-Trifluoroethane

The 3-MHz-resolution infrared spectra of the 10-μm bands of thegaucheconformer of 1,2-difluoroethane (HFC152) and theC1-symmetry conformer of 1,1,2-trifluoroethane (HFC143) have been measured using a molecular-beam electric-resonance optothermal spectrometer with a tunable microwave-sideband CO2laser source. For 1,2-difluoroethane, two bands have been studied, the ν17B-symmetry C–F stretch at 1077.3 cm−1and the ν13B-symmetry CH2rock at 896.6 cm−1. Both bands are well fit to a asymmetric-rotor Hamiltonian to better than 0.5 MHz. The ν13band is effectively unperturbed, while the ν17band is weakly perturbed, as shown by the large change in centrifugal distortion constants from the ground state values. Two bands have also been studied for 1,1,2-trifluoroethane, the ν11symmetric CF2stretch at 1077.2 cm−1and the ν13C–C stretch at 905.1 cm−1. One of the two bands, ν11, is unperturbed and fit to near the experimental precision. The ν13vibration, on the other hand, is weakly perturbed by an interaction with a nearby state. This perturbation leads to a doubling or splitting of the lines, due to a perturbation-induced lifting of the degeneracy of the symmetric and antisymmetric tunneling states associated with tunneling between the two equivalentC1forms. For theJ,Kastates studied, the splittings are as large as 37 MHz. Combining this observation with published low-resolution far-infrared measurements of torsional sequence-band and hot-band frequencies and calculations from an empirical torsional potential allows us to identify the perturbing state as ν17+ 6ν18. Here, ν17is the CF2twist and ν18is the torsion. The matrix element responsible for this interaction exchanges eight vibrational quanta!

Electric resonance optothermal spectrum of the 920 cm-1v14+v15torsional combination band of acetaldehyde

The 920 cm-1 vibrational band of acetaldehyde (CH3CHO) has been studied at about 2 MHz (FWHM) resolution using an electric resonance optothermal spectrometer and a tunable microwave-sideband CO2 laser. Microwave and radiofrequency infrared double resonance and precise combination differences are used to assign the spectrum and verify that it originates from the vibrational ground state of the molecule. The band is observed to have a large A-E torsional tunnelling splitting of 1·45 cm-1, with the A state higher in energy than the E state. The K′ = 1 and 2 levels for the E state have large K splittings of 28·8 GHz and 54·5 GHz, respectively, which are similar to the 37·3 GHz and 58·4 GHz splittings found for the first excited torsional vibration of the ground vibrational state. The above observations demonstrate that the 920 cm-1 vibrational state has one quantum of torsional excitation, confirming previous low-resolution assignments of the band to the A′ v 14 + v 15 combination vibration, consisting of the...

Frequency-resolved measurement of fast intramolecular vibrational energy redistribution (IVR) in the O–H stretch of gas-phase ethanol

The frequency-resolved, molecular-beam spectrum of the O–H stretch of ethanol near 3678 cm−1 has been measured at 10 MHz resolution using an electric-resonance optothermal spectrometer (EROS). A portion of the R branch of the predominantly a-type O–H stretch for the more stable trans conformer has been rotationally assigned using a high-sensitivity infrared-microwave double-resonance technique. The spectrum shows extensive near-resonant perturbations that characterize the intramolecular vibrational energy redistribution (IVR) process. Single rovibrational transitions of the O–H stretch normal mode are found to be fractionated into more than 60 transitions due to the coupling of the O–H stretch to the background of nearby vibrational states. The overall width of the collection of molecular eigenstates coming from a single zeroth-order rovibrational state in the vibrationally excited state gives a 25 ps lifetime for energy redistribution from the O–H stretch in trans-ethanol. This lifetime is in reasonable agreement with previous time-resolved measurements of the O–H stretch lifetime of 70 ps for ethanol in CCl4. Comparison of the two measurement techniques is discussed.

Molecular-beam spectrum of the 970 cm−1 Fermi triad of CF3CH3

A tunable microwave-sideband CO2 laser is used with an electric-resonance optothermal spectrometer to investigate the infrared spectrum of CF3CH3 near 970 cm−1. A Fermi-coupled triad of states is observed, resulting from the interactions of 2ν6+ν11 and ν5+ν12 with the fundamental vibration, ν10, which is assumed to carry the oscillator strength in this region. The high resolution (∼3 MHz) of the spectrometer allows the observation of tunneling splittings associated with the ν6 torsional vibration. These splittings are used to identify the torsional character of the states observed. At the normal-mode level the ν10 and ν5+ν12 states are found to be nearly degenerate and interacting by an anharmonic matrix element of ∼3 cm−1. The lower-energy component of this diad exhibits torsional splittings of up to 400 MHz due to an anharmonic coupling of 0.70 cm−1 with the lower energy 2ν6+ν11 state which has an intrinsic tunneling splitting of ∼800 MHz. A fourth state, 3ν6+ν12, which has a still larger zeroth-order tunneling splitting, may also be affecting the torsional splittings of the observed states. The present investigation illustrates the utility of using resolved torsional splittings to unravel complex vibrational couplings in molecules.

Perturbations in the infrared spectrum of the NH3 umbrella mode of HOH—NH3

A tunable microwave-sideband CO2 laser has been used with a molecular-beam electric-resonance optothermal spectrometer to observe the infrared spectrum of the NH3 umbrella fundamental vibration (ν5 in Cs ) of HOH––NH3 at a resolution of ∼3 MHz. Ground- and excited-state assignments were verified and extended using microwave–infrared double-resonance spectroscopy, with microwave transitions observed in both the ground and the excited states. The spectrum exhibits numerous perturbations, as evidenced by the observation of a minimum of 13 subbands originating from the (K,m)=(0,0) ground NH3 internal-rotor state and the (K,m)=(±1,±1) first excited NH3 internal-rotor state. For an unperturbed spectrum, only four such subbands are expected, two for the symmetric H2O tunneling state and two for the antisymmetric H2O tunneling state. The rotational progressions within the excited states are poorly fit to polynomial series in J(J+1), in contrast to the ground-state progressions which are well characterized by such series. The B rotational constants in the excited states are smaller than in the ground state, indicating an extension of the hydrogen-bonding interaction distance upon vibrational excitation. This is consistent with the observed infrared band origin for the (K,m)=(0,0) state of ∼1021 cm−1, which is blue shifted by 71 cm−1 from the hypothetical inversion-free 950 cm−1 ν2 band origin of uncomplexed NH3. The observed ν5 band origin is also in good agreement with matrix-isolation results scaled to correct for the matrix shift of the NH3 umbrella frequency found in the recently studied NH3–HCN complex. The complex does not dissociate upon vibrational excitation, implying that the binding energy is greater than the laser frequency of ∼1021 cm−1.

Optothermal-detected microwave-sideband CO 2-laser spectroscopy of NCH-NH 3

A microwave-sideband CO 2 laser is used together with an electric-resonance optothermal spectrometer to measure the infrared spectrum of the NH 3 umbrella vibration in NCH-NH 3 at a resolution of ≈ 3 MHz. The infrared radiation is produced by mixing Lamb-dip-stabilized CO 2 laser radiation with synthesizer-derived microwave radiation in a CdTe-buffered GaAs stripline electrooptic waveguide modulator. For NCH-NH 3 a symmetric top spectrum is observed with a band origin at 1041.7 cm −1, blue-shifted ≈ 91.8 cm −1 from the hypothetical inversion-free ν 2 band origin of free NH 3, which indicates a decrease in the van der Waals zero-point binding energy, D 0, for the excited state. The observed Δ B of −14.3 MHz, implying a hydrogen-bond extension, is consistent with this blue shift. The vibrationally excited complex does not predissociate within the ≈ 1 ms transit time between the laser excitation region and the bolometer detector, implying that D 0 is greater than the laser frequency, ≈ 1042 cm −1.

Infrared and microwave study of angular–radial coupling effects in Ar–HCN

Microwave and infrared spectra of Ar–HCN have been obtained using an electric-resonance optothermal spectrometer. The microwave measurements extend to higher J the previous results of Leopold et al. and Klots et al., allowing the determination of higher-order centrifugal distortion constants for this quasilinear, highly nonrigid complex. A Padé approximant fit to the microwave data indicates a significant rotation-induced asymptotic increase in the zero-point center-of-mass separation between the Ar and the HCN, above that expected from pure radial distortion. This results from the large coupling between the angular and radial degrees in the intermolecular potential forcing the centrifugal alignment of the HCN. Infrared spectra are reported for the C–H streching fundamental ν1 and the combination band ν1+ν15, where ν5 is the van der Waals bending vibration. The band-origin difference between these two bands gives ν5=7.8 cm−1, in rough agreement with the 10 cm−1 harmonic value predicted from the microwave-determined nuclear quadrupole coupling constant. The complexation-induced red shift of the C–H stretching vibration is 2.69 cm−1 and the vibrational predissociation linewidths Γ are <10 MHz (FWHM). The vibrationally excited complex predissociates before striking the bolometer detector, implying that the predissociation lifetime τ<1 ms.

Microwave and infrared electric-resonance optothermal spectroscopy of HF–HCl and HCl–HF

Microwave and infrared spectra of HF–HCl and HCl–HF have been obtained using a molecular-beam electric-resonance optothermal spectrometer, which operates by quadrupole-field focusing of polar molecules onto a bolometer detector. The HF–HCl microwave measurements extend to Ka=1, the previous Ka=0 results of Janda, Steed, Novick, and Klemperer, allowing the determination of the Ka dependence and asymmetry of the Cl quadrupole coupling constant. For the metastable HCl–HF isomer no previous spectroscopic measurements have been reported. Here, microwave spectra are observed for the Ka=0 and 1 states and interpreted in terms of an L-shaped hydrogen-bonded structure for the complex, with a 3.235 Å center-of-mass separation between the HF and HCl subunits. The DJ distortion constant indicates that the harmonic stretching force constant for HCl–HF is ∼35% larger than that of HF–HCl. Infrared spectra of the Ka=0–0 and 1–0 subbands of the H–F stretching band for HF–HCl and of the Ka=0–0 subband of the H–F stretch for HCl–HF are also reported. The vibrational predissociation linewidths depend on vibration, Ka state, isotopic species, and isomer excited.