Volume 81, Issue 10, 15 November 1984
Index of content:

The high resolution infrared spectrum and molecular structure of the superacid H_{2}F^{+} by velocity modulation laser absorption spectroscopy
View Description Hide DescriptionThe gas phase infrared spectrum of the fluoronium ion (H_{2}F^{+}) was recorded with Doppler‐limited resolution between 3080 and 3520 cm^{−} ^{1} by velocity modulation laser absorption spectroscopy of a hydrogen plasma containing 5% HF. One hundred and nine of the observed transitions were assigned to the symmetric stretch (ν_{1}) and 217 to the asymmetric (ν_{3}) stretch. A least squares analysis of ν_{3} transitions up to J=11 and K _{ a }=6 and ν_{1} transitions up to K _{ a }=4 with Watson’s S‐reduced Hamiltonian yielded rotational constants and centrifugal distortion constants for the vibronic ground state and for the two excited states. Small Coriolis interactions between ν_{1} and ν_{3} levels were observed, but were not treated explicitly. The band origins for ν_{3} and ν_{1} are 3334.6895(26) and 3348.7078(36) cm^{−} ^{1}, respectively. The r _{0} structure of H_{2}F^{+} was determined as: r _{HF}=0.9577 Å <HFH=114.55°.

Spectroscopic manifestation of intramolecular relaxation of azulene in supersonic jets
View Description Hide DescriptionIn this paper we report on the line profiles in the absorption spectra for the S _{0} → S _{1} electronic origin and for the 659 and 1394 cm^{−} ^{1} vibrational excitations in the S _{1}manifold of azulene in pulsed, planar, supersonic expansions. The S _{1} electronic origin exhibits a Lorentzian line shape, whose homogeneous linewidth results in the lifetime τ=0.8±0.2 ps for interstate electronic relaxation in the isolated molecule.

Temperature‐dependent ESR hyperfine constants for nitroxides and orientational correlation time determination
View Description Hide DescriptionA direct experimental observation of temperature‐dependent electron‐spin‐resonance (ESR) hyperfine coupling constants has been made for TANOL nitroxide radicals in a polycarbonate polymer. This observation is based on the accurate rigid‐limit spin Hamiltonian constants obtained from the ESRpowder spectra of deuterated TANOL enriched with ^{1} ^{5}N isotope (I=1/2) (TANOLD–^{1} ^{5}N) at 77 K, and the isotropic hyperfine splitting factor obtained from the completely motionally shifted ^{1} ^{5}N ESR hyperfine lines at temperatures >445 K. From this reduction in the isotropic hyperfine splitting, it is determined that the hyperfine splitting factor ‖A _{ z }/g _{ z }μ_{ B }‖ is reduced by 0.16 and 0.11 mT for TANOL–^{1} ^{5}N and TANOL–^{1} ^{4}N, respectively, when the temperature is increased from 77 to 445 K. The latter value for normal TANOL–^{1} ^{4}N is consistent with the temperature‐dependent hyperfine splitting factor ‖A _{ z }/g _{ z }μ_{ B }‖ deduced recently from the application of a new theoretical and experimental approach for studying slow‐tumbling nitroxide radicals. It is shown that the temperature‐dependent nitroxide hyperfine coupling constants will directly affect the measurements of slow orientational motional correlation times by conventional methods.

Direct absorption spectroscopy of jet‐cooled polyenes. I. The 1 ^{1} B ^{+} _{ u }←1 ^{1} A ^{−} _{ g } transition of t r a n s, t r a n s‐1,3,5,7‐octatetraene
View Description Hide DescriptionThe direct absorptionspectrum of the 1 ^{1} B ^{+} _{ u }←1 ^{1} A ^{−} _{ g } transition of jet‐cooled t r a n s,t r a n s‐1,3,5,7‐octatetraene is reported here for the 35, 120–42, 600 cm^{−} ^{1} region. The alleviation of vibrational sequence band congestion in the jet spectrum results in an order‐of‐magnitude decrease in the observed breadth of the origin band and a comparable sharpening throughout the spectrum, enabling the redetermination of vibronic band positions, relative intensities, and widths. Vibrational assignments for the seven observed active modes are discussed, and results are compared to the predictions of previously reported vibronic structure calculations.

Direct absorption spectroscopy of jet‐cooled polyenes. II. The 1 ^{1} B ^{+} _{ u }←1 ^{1} A ^{−} _{ g } transitions of butadienes and hexatrienes
View Description Hide DescriptionIn the present paper, we report the direct absorption spectra of the 1 ^{1} B ^{+} _{ u }←1 ^{1} A ^{−} _{ g } transitions of gas phase butadiene, deuterated and methylated butadienes, and the c i s and t r a n s isomers of hexatriene cooled to low rotational and vibrational temperatures in supersonic molecular jets. These jet absorption spectra allow the more accurate determinations of Franck–Condon factors, upper state vibrational intervals and vibronic band homogeneous widths. We discuss the experimental constraints that the measurements reported here and in the previous paper of this series impose on theoretical models of the equilibrium structures and relaxation dynamics of the 1 ^{1} B ^{+} _{ u }excited states of the small linear polyenes.

Jahn‐Teller EPR spectra of Cu^{2} ^{+} in MgSiF_{6}⋅6H_{2}O
View Description Hide DescriptionThe 34 GHz EPR spectrum of Cu^{2} ^{+} in MgSiF_{6}⋅6H_{2}O showed a ‘‘static’’ Jahn–Teller effect at 4.2 K with two inequivalent Jahn–Teller sites per unit cell. The six axially symmetric sets of Cu^{2} ^{+} lines had their z axes parallel to the three tetragonal axes of two cubes, which were rotated by approximately 40° with respect to each other about a common [111] axis, which is the crystal c axis. The measured spin‐Hamiltonian parameters at 4.2 K for each set of lines were g _{∥}=2.47±0.01, g _{⊥}=2.10±0.01, and ‖A _{∥}‖=(110±3)×10^{−} ^{4} cm^{−} ^{1}. There was a gradual decrease in the anisotropy of the spectrum on warming the crystal, with a single, nearly isotropic line being observed above 220 K. At 270 K the spectrum had axial symmetry about the c axis with g ^{′} _{∥} =2.23±0.01 and g _{⊥} =2.25±0.01. The temperature evolution of the spectrum was interpreted in terms of a Boltzmann distribution over inequivalent distorted Jahn–Teller configurations, with one potential well lowered by an amount Δ≊105 cm^{−} ^{1} below the other two.

A Raman study of pressure‐densified vitreous silica
View Description Hide DescriptionWe have obtained polarized Raman spectra for vitreous silica densified at 3.95 GPa and 530 °C. The glass spectra show considerable changes on densification, which are discussed in terms of the observed reduction in average intertetrahedral bond angle and the central force model for connected networks. The results of this densification experiment are compared with previous work at high pressure but room temperature. This comparison suggests a model for the densification mechanism, and indirectly supports recent structural interpretations of the Raman spectrum of fused silica.

Effect of deuterium substitution on the triplet state dipole moments of substituted benzaldehydes
View Description Hide DescriptionThe laser induced phosphorescence excitation spectra of 2, 4, 5‐trimethylbenzaldehyde ‐1h _{1} and‐1d _{1}, and 2, 5‐dimethylbenzaldehyde ‐1h _{1} and ‐1d _{1} isolated in durene single crystals have been obtained, and the dipole moment difference vectors associated with the bands identified, from their intensities and positions in the spectra, as the T _{1} and T _{2} origins have been measured. Since the data indicate that the T _{2} origin is mixed with T _{1} vibronic levels, the symbol T _{2} is used only as a convenient label. The T _{2} band of TMB shifts 26 cm^{−} ^{1} to the red upon deuteration, and the corresponding band in DMB shifts 36 cm^{−} ^{1} to the blue. The magnitude of the dipole moment difference vector found for this band is +1.2 D in TMB and −2.0 D in DMB. The spectral shifts upon deuteration and these dipole moment difference values both are consistent with the T _{2} band in TMB being predominantly ππ* in character and the T _{2} band in DMB being predominantly nπ*. Upon deuteration, the dipole moment difference vector decreases for TMB and appears to increase for DMB. These variations may indicate that the coupling of the T _{2} origin with the T _{1} vibronic levels decreases in TMB and increases in DMB upon substitution of deuterium for the aldehydic hydrogen. No deuterium isotope effect on the dipole moment difference vectors for the T _{1} origin bands of these molecules was observed.

Rotational collisional narrowing in the NO fundamental Q branch, studied with cw stimulated Raman spectroscopy
View Description Hide DescriptionSelf‐broadened NO Q‐branch spectra were obtained in the pressure region ∼20–100 kPa. We determined J‐ and Ω‐dependent pressure broadening coefficients. The observed collisional narrowing was fitted by means of a relaxation matrix theory, incorporating recent experimental and theoretical values of NO state‐to‐state rates. A ‘‘fitting law’’ representation of the state‐to‐state rates yielded good agreement with both the measured broadening coefficients and the observed spectrum.

Synchrotron radiation study of vibrationally resolved partial photoionization cross sections of CO_{2} between 64 and 80 nm
View Description Hide DescriptionVibrationally resolved partial photoionization cross sections of CO_{2} have been measured for the X̃ ^{2}Π_{ g } ionic state for wavelengths between 64 and 80 nm and for Ã ^{2}Π_{ u } and B̃ ^{2}Σ^{+} _{ u } from 64 to about 2 nm below the respective thresholds. The autoionization of the Rydberg series converging to Ã and those converging to B̃ is discussed in terms of population of two vibrational levels of the ground electronic state of the ion. The Rydberg series with limit at C̃ ^{2}Σ^{+} _{ g } (000) are shown to decay mainly to B̃ ^{2}Σ^{+} _{ u } in its (000) and (100) vibrational levels and to a lesser manner to X̃ ^{2}Π_{ g }. They do not populate the Ã ^{2}Π_{ u } state.

Single‐crystal EPR study of bromobis (diethyldithiocarbamato) iron (III) diluted in the lattice of bromobis (diethyldithiocarbamato) arsenic (III)
View Description Hide DescriptionThe single‐crystal EPR spectra of Fe(S_{2}CNEt_{2})_{2}Br diluted in As(S_{2}CNEt_{2})_{2}Br have been recorded at liquid–helium temperature. They display a typical rhombic pattern with g ^{′} _{ X } =3.07(2), g _{ Y } =5.08(2), g ^{′} _{ Z } =1.95(2) in agreement with a ground Kramers doblet, ‖±〉≂‖S=3/2, M _{ S } =±1/2〉. The signals exhibit ligand hyperfine structure from the^{7} ^{9} ^{,} ^{8} ^{1} Br ions. Due to the overlapping of four, symmetry related signals, only a partial analysis of this hyperfinetensor was achieved. It was found that the ligand A‐tensor anisotropy is not pronounced and that the magnitudes of the three principal components lie in the range 80–100×10^{−} ^{4} cm^{−} ^{1}.

High resolution infrared spectroscopy of the ν_{1} (NH stretch) and ν_{2} (CH stretch) bands of HCNH^{+}
View Description Hide DescriptionThe infrared spectrum of the ν_{1}‐band (NH stretch) and the ν_{2}‐band (CH stretch) of protonated hydrogen cyanide HCNH^{+} has been observed based on recent theoretical predictions. This is the first observed spectrum of this ion in any wavelength region. From a least‐squares analysis of the rovibrational bands, the following spectroscopic constants have been obtained (in cm^{−} ^{1}): ν_{1} (NH stretch) ν_{2} (CH stretch) ν_{0} 3482.844 1(10) 3187.863 8(4) B _{1} 1.228 633(39) 1.228 494(19) B _{0} 1.236 024(37) 1.236 067(18) D _{1} 1.574(43)×10^{−} ^{6} 1.610(22)×10^{−} ^{6} D _{0} 1.596(42)×10^{−} ^{6} 1.620(21)×10^{−} ^{6} It is hoped that the rotational constants determined in this paper will lead to the identification of the microwave spectrum of this molecule in interstellar space.

Determination of the conformational barriers to internal rotation of 3‐fluoropropene from far infrared and low frequency Raman spectra
View Description Hide DescriptionThe far infrared (200–40 cm^{−} ^{1}) and low frequency Raman (1000–20 cm^{−} ^{1}) spectra of gaseous 3‐fluoropropene have been recorded. The fundamental asymmetric torsion for the conformer which has the fluorine atom c i s to the double bond has been observed at 164.62 cm^{−} ^{1} with four excited states falling at lower frequencies, and the corresponding fundamental torsion of the g a u c h e conformer was observed at 108.00 cm^{−} ^{1} with three excited states observed at lower frequencies. From these data the potential function for internal rotation of the asymmetric top has been determined and the following potential constants have been evaluated: V _{2}=459±29, V _{3}=830±9, V _{4}=18±8, and V _{6}=−37±5 cm^{−} ^{1}, with a ΔH of 304±20 cm^{−} ^{1} (869 cal/mol). It has been determined that the c i s conformer is the predominant form at ambient temperature in the gas phase and, from a temperature study of the Raman spectrum in this phase, the enthalpy difference between the c i s and g a u c h e conformers was determined to be 263±25 cm^{−} ^{1} (752 cal/mol). The calculated c i s to g a u c h e, g a u c h e to g a u c h e, and g a u c h e to c i s barriers are 1201 cm^{−} ^{1} (3.43 kcal/mol), 498 cm^{−} ^{1} (1.42 kcal/mol) and 869 cm^{−} ^{1} (2.48 kcal/mol), respectively. An alternative potential function was also calculated utilizing the successive g a u c h e asymmetric torsional transitions proceeding to higher frequencies. A second series of asymmetric torsional transitions were observed for the c i s conformer which are believed to be due to ‘‘hot bands’’ associated with the first excited state of the CCC bend. Additionally, each of the torsional transitions for the c i s conformer exhibits multiple splitting which is believed to be due to excited states of the CCF bend. These results are compared to similar quantities in some related molecules.

Infrared double resonance of fluoroform‐d with a tunable diode laser
View Description Hide DescriptionAn infrared double‐resonance experiment has been carried out on fluoroform‐d. The ν_{2}←0 and ν_{5}←0 fundamentals have been reanalyzed at a resolution of 0.001 cm^{−} ^{1}. From excited‐state absorptions in the ν_{2}+ν_{5}←ν_{5} and 2ν_{5}←ν_{5} bands, we obtain values for the anharmonicity constants x _{2} _{5}=−1.0 cm^{−} ^{1}, x _{5} _{5}=−0.23 cm^{−} ^{1}, and g _{5} _{5}=0.13 cm^{−} ^{1}. Both of the x _{ k5} values are in good agreement with Dennison’s rule estimates of Kirk and Wilt. Collision‐induced double‐resonance signals indicate propensity rules for rotational state changes ΔJ=±1, ΔK=±3n, where n may take integer values as large as 6 or 8. The rotational relaxation rates are three to five times hard‐sphere gas kinetic for polar collision partners (CDF_{3} itself or CH_{2}F_{2}), but only one‐fifth gas kinetic for helium collisions. These results are used to assess proposed models for multiple infrared photondissociation in CDF_{3} and for submillimeter‐wave optically pumped laser action in this gas.

The vapor phase far‐infrared spectra of 1H‐indene, benzo[b]furan, 1,3,2‐benzodioxaborole, and indole: Evidence for planar skeletons and kinetic anharmonicity in the out‐of‐plane ring deformations
View Description Hide DescriptionThe far‐infrared spectra of the vapors of 1H‐indene, benzo[b]furan, 1, 3, 2‐benzodioxaborole (catechol borane), and indole are reported in the region of 100–500 cm^{−} ^{1}. The spectra of the first three compounds exhibit series of C‐type Q branches for the three out‐of‐plane ring deformations, establishing planar skeletal configurations and indicating that the aromatic skeletons are not truly rigid. The lower of these two sequences are assigned as interlocking transitions arising from the deformation of the five‐membered ring and the butterfly mode, and are analyzed by means of a two‐dimensional Hamiltonian which includes for each mode a quadratic potential constant and a quadratic dependence of the reduced mass on each displacement coordinate, as well as a potential constant coupling the two modes.

The dependence of the ^{1} ^{3}C and the ^{1}H nuclear magnetic shielding on bond extension in methane
View Description Hide DescriptionThe mean bond displacements 〈Δr〉 in the methane isotopic homologous series ^{1} ^{3} ^{/} ^{1} ^{2}CX_{4−n }Y_{ n } (X, Y=H, D, T) at 300 K, and the temperature dependence of 〈Δr _{CH}〉 in ^{1} ^{3}CH_{4} from 250 to 350 K were calculated. With the assumption that the linear terms are sufficient to account for the isotope shifts, we determine from the ^{2} ^{/} ^{1}H‐induced ^{1} ^{3}C isotope shift an empirical value of(∂σ^{C}/∂Δr _{CH})_{ e }=−35±3 ppm/Å. This predicts a temperature dependence in the ^{1} ^{3}C resonance in CH_{4} gas in the zero‐pressure limit of 2.7×10^{−} ^{2} ppm over 100 °, which explains why it could not be observed. We observed the ^{1} ^{3} ^{/} ^{1} ^{2}C‐induced ^{1}H isotope shift in CH_{4}, −0.0024 ppm. With the same mean bond displacements, this isotope shift gives an estimate of (∂σ^{H i } /∂Δr _{CH i } )_{ e }=−38±3 ppm/Å. From the reported ^{2} ^{/} ^{1}H‐induced two‐bond ^{1}H isotope shift in CH_{4}, −0.016 ppm, using the mean bond displacements and the derivative obtained from the ^{1} ^{3} ^{/} ^{1} ^{2}C‐induced one‐bond isotope shift, we get an estimate of (∂σ^{H i }/ ∂Δr _{CH j }) _{ e }=−1.3±0.2 ppm/Å.

The additivity of NMR isotope shifts
View Description Hide DescriptionOne of the most interesting and useful aspects of the isotope effect on nuclear magnetic shielding is the proportionality of the shift to the number of substituted atoms in equivalent positions. In this paper we show the quantitative basis for the additivity of isotope shifts in NMR, using the CX_{4−n }Y_{ n } (X,Y=H,D,T) system and the linear triatomic systems CO_{2}, NNO, and OCS as examples. We also predict small deviations from additivity and find that these deviations are consistent with those observed for ^{1} ^{4}N shifts in the NH_{4−n }D^{+} _{ n } homologous series. Furthermore, we determine the mass dependence of the one‐bond isotope shift.

The NMR isotope shift in polyatomic molecules. Estimation of the dynamic factors
View Description Hide DescriptionIn a continuing study of the factors which determine the isotope shifts in NMR we consider here substitution of end atoms in molecular types in which the bond angle deformation does not play a significant role. We show that in molecules of the type AX_{ n } the rovibrational correction to shielding of nucleus A can be obtained directly from the isotope shift without dynamical calculations. We propose a method for estimating 〈Δr〉, knowing only the equilibrium bond length, the masses, and the rows of the Periodic Table of the atoms in the bond. We test this on diatomic molecules for which we are able to calculate 〈Δr〉 directly from the spectroscopic constants and examine its dependence on bond order. We apply the estimation method to polyatomic molecules for which we have completed a full dynamical calculation using the best available force fields. We use the estimated 〈Δr〉 for other molecular systems and obtain estimates of the shielding derivatives from the observed isotope shifts. The results compare well with derivatives that are known for related electronic environments.

High pressure Raman study of intermolecular interactions and Fermi resonance in liquid ethylene carbonate
View Description Hide DescriptionThe Raman band of the ring vibration (ν_{5}) and the Fermi resonance between the carbonyl stretching (ν_{2}) and the first overtone of the ring breathing vibration (2ν_{7}) have been studied in liquid ethylene carbonate in the pressure range between 1 and 3000 bar and at temperatures from 40 to 160 °C. The relative changes of the transition dipole moments of both bands in resonance are estimated from the measurements of their intensity ratios. The knowledge of these parameters enables a comparison of the experimental frequency noncoincidence effect between isotropic and anisotropic components of the bands and the theory based on strong dipole moment coupling. The noncoincidence effect for the ν_{5} vibration are also explained in terms of this theory. The pressure induced frequency shifts of the bands are interpreted qualitatively by the permanent dipole–dipole coupling model.

A picosecond holographic grating approach to molecular dynamics in oriented liquid crystal films
View Description Hide DescriptionThe picosecond transient grating technique offers a new approach to the characterization of rotational dynamics and mechanical properties of thin liquid crystalfilms. Sample excitation by two crossed 100 ps pulses having parallel polarization results in two kinds of phase gratings: one due to the optical Kerr effect, and the other to a standing longitudinal acoustic wave. Rotational reorientation times are calculated from the relaxation of the Kerrgrating, while the ultrasonic velocity and absorption are obtained by monitoring the acoustic response. If the excitation pulses are perpendicularly polarized, no longitudinal acoustic waves are generated, so that the signal is due exclusively to the Kerr effect. Whereas previous workers using ∼20 ns excitation pulses observed a single exponential Kerr relaxation in the isotropic phase, we are able to resolve the decay into a fast nonexponential component followed by a slow exponential component. While the slow component disappears below the isotropic→nematic transition, we can detect the fast component even below the nematic→smectic A transition in CBOA. An explanation of the fast component is proposed involving individual rather than collective molecular reorientation. The nature of the polarizationgrating resulting from perpendicularly polarized excitation pulses is described. An unusual property of this grating is that it acts like a half‐wave plate for the diffracted signal. Thus the polarizations of the incoming probe and the outgoing diffracted pulses can be made orthogonal. The theory and implications of this result are discussed, and the extension of the transient grating technique to the study of model biological membranes is outlined.