T. D. Klots

Relaxation of conformers and isomers in seeded supersonic jets of inert gases

We have studied the relaxation of conformers and the formation/relaxation of isomeric, weakly bonded dimers in pulsed supersonic expansions of seeded inert gases (He, Ne, Ar, Kr). The relaxation was determined from the intensity of a rotational transition for the higher energy species as a function of carrier gas composition, using the Balle/Flygare Fourier transform microwave spectrometer. Of thirteen molecules with rotational conformers which we examined, those with barriers to internal rotation greater than 400 cm−1 did not relax significantly in any of the carriers. The higher energy forms of ethyl formate, ethanol, and isopropanol, with smaller barriers, were not relaxed by He; those of ethanol and isopropanol were somewhat relaxed by Ne; and all were completely relaxed by as little as 5 to 20 mole percent of Ar or Kr in He or Ne. The relaxation in He or Ne is first order in the concentration of added Ne, Ar, or Kr as well as in the concentration of the high energy conformer. The pseudo first‐order r...

Microwave rotational spectra, hyperfine interactions, and structure of the hydrogen fluoride dimers

The hyperfine structure has been resolved for the two J=0→1 transitions of (DF)2, for the low frequency J=0→1 transition of (HF)2, and for the J=0→1 transition of the mixed species HFDF. Also, the J=0→1 transition and hyperfine structure were observed for the other mixed species DFHF, not found previously. The relative intensities of the transitions for HFDF and DFHF, and their zero‐point vibrational frequencies, correspond to an effective equilibration temperature between the two species of ∼50 K in the gas expansion. Resolution and assignment of the (DF)2 and (HF)2 hyperfine structure was aided by a method for suppressing the Doppler doubling in a Flygare spectrometer; it improved resolution twofold to a FWHM of 5 kHz at 11–14 GHz. The results bear on several features of the angular structure of this unusual system. At equilibrium, the end H/DF in HFDF and DFHF is bent 60±2° from the F⋅⋅⋅F axis, while the H/DF in the hydrogen bond is bent 7±3° in the opposite direction. The tunneling in (DF)2 gives an a...

Rotational spectra and structures of the Ar2–H/DF trimers

The feasibility of studying weakly bound, neutral van der Waals clusters through their microwave rotational spectra has been shown by identifying and characterizing Ar2–H/DF. Detection of the trimeric complexes was accomplished with the Flygare Mark II Fourier transform spectrometer, modified to better utilize its inherent sensitivity. Twenty‐six b‐dipole transitions (all ee↔oo) were observed in the 3–18 GHz region for Ar2–HF, and 19 for Ar2–DF, and their hyperfine structure analyzed. Fitting of the line centers with Kirchhoff’s NBS program gave values (all in MHz) for Watson’s determinable parameters of 3576.508(1), 1739.139(1), and 1161.054(1) for the rotational constants A‘, B‘, and C‘ in Ar2–HF, and of 3506.791(1), 1744.056(1), and 1155.636(1) in Ar2–DF. The centrifugal distortion constants τ1, τ2, τaaaa, τbbbb, and τcccc were found to be −0.2039(3), −0.0522(1), −0.5353(5), −0.1159(1), and −0.02021(1) in Ar2–HF, and −0.2116(7), −0.0528(2), −0.4941(7), −0.1123(1), and −0.0191(1) in Ar2–DF. Three small ...

Rotational spectra and structures of the Ar2–H35Cl/37Cl trimers

Microwave rotational spectra have been observed for both Cl isotopes of the Ar2–HCl trimer with the pulsed nozzle Fourier transform method using the Flygare Mark II spectrometer. The Cl nuclear quadrupole hyperfine structure was analyzed for each of the transitions and the coupling constants and line centers determined. Sixteen transitions were observed in the 2 to 15 GHz region for the 35Cl species and 11 for 37Cl. The line centers were fitted to obtain ground state rotational and quartic centrifugal distortion constants A‘, B‘, C‘, τ1, τ2, τaaaa, τbbbb, and τcccc. For Ar2–H35Cl, the values are 1733.857, 1667.932, 844.491, −0.1170, −0.0292, −0.1199, −0.0802, and −0.0079 MHz, respectively, and for Ar2–H37Cl: 1733.824, 1606.877, 828.497, −0.1121, −0.0279, −0.1205, −0.0737, and −0.0075 MHz. The equilibrium geometry is determined to be T shaped with C2v symmetry and the H end of the HCl closest to the Ar2. Large amplitude slightly anisotropic torsional motion of the HCl is evident from the hyperfine constant...

Rotational spectra, structure, Kr‐83 nuclear quadrupole coupling constant, and the dipole moment of the Kr‐benzene dimer

Rotational spectra were observed for the Kr‐82, Kr‐83, Kr‐84, and Kr‐86 isotopic species of the Kr‐benzene dimer with the Mark II Balle‐Flygare Fourier transform microwave spectrometer. The spectra are those of a symmetric top, with the Kr on the symmetry axis of the benzene. In the most abundant Kr‐84 benzene dimer the rotational constants B0, DJ, and DJK were found to be 795.6821(1) MHz, 1.315(1) kHz, and 7.895(4) kHz, respectively. R, the Kr to benzene center‐of‐mass distance, is 3.663 A and the zero‐point vibrational bending of the benzene with respect to R gives an average angular displacement of 16.6°. The Kr‐83 quadrupole coupling constant was determined experimentally to be −5.201(5) MHz and the dipole moment, 0.136(2) D. A theoretical prediction of these electrically induced properties gives results for a distributed multipole model which are superior to those using central multipoles.

J dependence of χa(14N) for the Ar–HCN dimer: Coupling of stretching and bending in the potential function

The previously reported rotational spectrum of Ar–HCN [J. Chem. Phys. 81, 4922 (1984)] has shown the weakly bound dimer to be highly nonrigid. Superficially linear, the dimer has several anomalies, including large centrifugal distortion and an unexpectedly large bending amplitude of the HCN. We here describe high‐resolution rotational spectra which identify another anomaly. The 14N hyperfine interaction constant of the dimer increases linearly with J(J+1) for Ar–HCN, 36Ar–HCN, and Ar–DCN, indicating a decrease in the average HCN bending amplitude (θ). For Ar–HCN this is from 30.97° for J=0 to 30.17° for J=5. At the same time, there is an increase in the average Ar to HCN c.m. separation R from 4.3433 to 4.3496 Ar. The cause of this behavior and of the other anomalies is found to be the shape of the potential function as calculated with a largely classical electrical model employing low‐order moments and multipole polarizabilities. The calculated potential surface exhibits strong coupling between radi...

Rotational spectrum and structure of the linear CO2–HCN dimer: Dependence of isomer formation on carrier gas

A linear hydrogen‐bonded dimer, OCO–HCN, has been identified and characterized via its microwave rotational spectrum. The study was made using the pulsed nozzle Fourier transform method with the Flygare/Balle Mark II spectrometer. A T‐shaped HCN–CO2 dimer was reported earlier by the Klemperer group. Rotational constants have been determined for all seven monoisotopically substituted species of the linear form. B0 , DJ , and χaa (14 N) for the normal isotopic dimer are 1057.9397(2) MHz, 1.372(8) kHz, and −4.2466(5) MHz, respectively. The average torsional displacements of the OCO and HCN monomers about their center of mass (c.m.) are found to be 7.66° and 12.40°, based on the substitution O–C and C–N bond distances for the dimer. With these values for α and γ, the B0 for the normal isotopic dimer corresponds to a c.m. to c.m. distance R=5.035 A. Bending and stretching force constants and the well depth (e∼590 cm−1 ) are estimated from the centrifugal distortion. The relative concentrations of the linear an...

Rotational spectra and structures of small clusters: The Ar4–H/DF pentamers

The 40Ar4–HF and –DF clusters have been identified and characterized by their microwave rotational spectra using a Flygare–Balle FT spectrometer with a pulsed supersonic nozzle to generate and detect the clusters. The observed J=2→3 to 6→7 transitions have fine structure limited to K=0, ±3 and ±6, indicative of symmetric tops with a threefold axis of symmetry produced by I=0 nuclei. For Ar4–HF the rotational constants B0, DJ, and DJK are 623.4539(1) MHz, 1.062(1) kHz, and 0.611(31) kHz, respectively; and for Ar4–DF, 618.5846(1) MHz, 1.030(1) kHz, and 0.665(4) kHz. The results show that the clusters consist of the trigonal Ar3–H/DF tetramer reported earlier, with a fourth argon on the back side of the Ar3 group to form a tetrahedral or near tetrahedral Ar4. In both clusters the H/DF lies along the threefold axis of the Ar3 group, with the H/D end pointed at its face. If one assumes an Ar–Ar distance of 3.85 A for both clusters, the Ar3 plane to HF c.m. distance for Ar4–HF with a tetrahedral Ar4 is 2.713 A ...

Rotational spectra and structures of the OC– and H3N–HCN–HF trimers: Coaxial mixing nozzle for reactive species

Rotational spectra are reported for several isotopic species of the OC–and H3N–HCN–HF heterotrimers, detected with a pulsed nozzle, Fourier transform, Balle/Flygare microwave spectrometer.Rotational constants for the main isotopic species of the OC trimer are a B 0 of 615.574 MHz and D J of 251 Hz, and for H3N, a symmetric top, a B 0 of 1067.161 MHz and D J and D J K of 0.40 and 63 kHz. Their structures are composites of those reported for the X–HCN and HCN–HF dimers. They are effectively axially symmetric but have some shrinkage from the distances in the dimers. The shrinkages found in r 1, the c.m. to c.m. distance for X–HCN are 0.070 and 0.098 A for X=OC and H3N, respectively, and in r 2 for HCN–HF, 0.033 and 0.027 A. The 1 4N and H–F hyperfine interactions in OC–HCN–HF are the same as those reported for the HCN–HF dimer. Detection of the X=OC and H3N trimers out of the many species possible required care in their generation. Both were favored by the strongly bonded HCN–HF subunit. The OC–HCN–HF was further enhanced by using a high concentration of CO in the gas expansion. For H3N–HCN–HF a coaxial mixing nozzle was developed to avoid the formation of NH4F(s). The selectivity and simplicity of the nozzle should be helpful in extending the range of species observable with pulsed nozzles.

Experimental separation of torsional and charge redistribution effects in rotational spectra of HCN dimers

Abstract Boand Djhave been determined for H13C15N-HC15N, HC15N-H13C15N, HC15N-DC14N and DC13N-HC15N. From them and other, previous results a full substitution structure has been obtained for HCN(1)-HCN(2). It leads to torsional amplitudes θ1 and θ2 of 13.6 and 9.3° for the two monomers in the dimer. A determination by fitting Bo for six isotopic species gives 13.7 and 8.7°. These values are used to separate torsional and charge redistribution effects upon the hyperfine interactions of 14N and D in the dimers. For 14N, about 40% of the difference in χa between HCN monomer and dimer is caused by charge redistribution. The C-D bond length in the dimer is considered.