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 ??aa (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 ?? and ??, the B0 for the normal isotopic dimer corresponds to a c.m. to c.m. distance R=5.035 ??. Bending and stretching force constants and the well depth (?????590 cm???1 ) are estimated from the centrifugal distortion. The relative concentrations of the linear and T???shaped isomers are unusually sensitive to the carrier gas used in the supersonic jet expansion. The linear form could not be detected at all with argon as the carrier gas but gave a strong signal in neon first run (70% Ne, 30% He). In contrast, the T form gave strong signals in both carrier gases. However, a carrier???gas effect was not found for the N2 O/HF dimer pair, which has a high barrier between the bent NNO???HF and linear FH???NNO isomers. Similar results were obtained for chlorocyclohexane (CCH) and ethyl formate (EF), which have two conformational isomers. In CCH which has a high barrier to a???e interconversion, the two conformers gave strong signals in both Ar and He. In EF, with a low barrier, the gauche conformer could not be detected in Ar but gave a strong signal in He, while the trans form gave strong signals in both carrier gases

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 rate constants (larger in Ne than in He) increase sharply with Z of the rare gas, roughly in a 0:1:2:4 progression for He, Ne, Ar, and Kr, suggesting that the relaxation involves relatively long???range polarization effects. Similar behavior was found in the formation/relaxation of the weakly bonded dimer pairs: linear OCO???HCN, T???shaped HCN???CO2; linear FH???NNO and bent NNO???HF; and bent HF???DF and DF???HF. The case of the HCN/CO2 dimers is particularly striking. The T???shaped dimer was found first, using Ar as the carrier gas. Five years later the linear form was found with first run neon as carrier, but it could not be detected at all with Ar as the carrier. These results show that in favorable cases high energy species can be studied in supersonic expansions by freezing out a ??????high???temperature?????? concentration with a nonrelaxing carrier gas

ROTATIONAL SPECTRUM AND STRUCTURE OF THE LINEAR HCN TRIMER

Microwave rotational spectra have been observed for 22 isotopic species of an HCN, hydrogen???bonded trimer with the pulsed nozzle, Fourier transform method using the Flygare/Balle Mark II spectrometer. The 14N nuclear quadrupole hyperfine structure was analyzed and the interaction constants and line centers determined. The line centers were fitted to obtain ground vibronic state rotational constants. For the normal isotopic species of (HCN)3, B0 was found to be 469.3073(1) MHz and DJ, 82.6(1) Hz. The quadrupole coupling constants ??(n) are ???4.049(2), ???4.251(2), and ???4.375(1)MHz for n=1, 2, and 3, respectively, in HCN(1)HCN(2)HCN(3). The trimer has a linear or very near linear equilibrium structure. The B0???s are insensitive to the position and torsional oscillations of the central HCN but they determine the outer HCNs quite accurately. An isotopic substitution method gives R, the c.m. distance between the outer HCN???s, to be 8.790 ?? in the 14???14???14 species. A slightly smaller value 8.788 ?? is obtained from a fit of the B0???s which includes the effects of isotopic substitution on the H/D???C and C???N bond lengths in the monomer. The distance between c.m.(1) and c.m.(2) may be somewhat shorter (???0.01 ??) than that between c.m.(2) and c.m.(3). The average c.m. separation in the trimer (4.395 ??) is halfway between that of the dimer (4.447 ??) which is known to be linear and the infinite linear chains in the solid (4.34 ??). It is shown that the smaller quadrupole coupling constants in the trimer compared to the monomer are caused by a combination of torsional and charge redistribution effects. The torsional vibrations have average angular displacements of 12.60?? and 8.55?? for HCN(1) and HCN(3). The charge redistribution effect upon ?? in the trimer (6.5%) is about 1.8 times that in the HCN dimer. The dipole moment of the trimer was found to be 10.6 D, a value enhanced by 1.8 D over the vector sum of the three vibrating monomers (8.83 D), an enhancement about 1.7 times that of the dimer

ROTATIONAL SPECTRA AND STRUCTURES OF SMALL CLUSTERS CONTAINING THE HCN DIMER - X-(HCN)2 WITH X = CO, N-2, NH3, AND H2O

ABSTRACT This work is the counterpart of a previous report on the (HCN)2???Y trimers with Y=HF, HCl, HCF3, and CO2 [J. Chem. Phys. 90, 4069 (1989)]. Rotational spectra have been observed for several isotopic species of the OC???, N2???, H3N???, and H2O???(HCN)2 trimers, using a pulsed nozzle, Fourier transform Balle/Flygare microwave spectrometer. The structures are basically composites of those reported for the (HCN)2 and X???HCN dimers. The trimers are effectively axially symmetric, but have some shrinkage of dimensions. Rotational constants found for the main isotopic species of each trimer are: For X=OC, a B0 of 421.142 MHz and DJ of 110 Hz; for X=N2, 435.573 MHz and 155 Hz; for X=H3N, a symmetric top, a B0 of 675.777 MHz, DJ of 180 Hz, and DJK of 41.1 kHz; and for X=H2O, with C2v symmetry, a (B0+C0)/2 of 667.028 MHz, (B0???C0)/2 of 0.617 MHz, DJ of 173 Hz, and a DJK of 62.9 kHz. The rotational constants for the isotopic species of each trimer were used to determine the distances r1 and r2 between the centers of mass (c.m.) of adjacent monomers, r1 being that for X???HCN and r2 that for (HCN)2. For X=OC, N2, H3N, and H2O the shrinkages found in r1 are 0.068, 0.056, 0.084, and 0.074 ??, respectively, and in r2 0.013, 0.013, 0.044, and 0.026 ??. The 14N quadrupole coupling constants were determined by selective 15N substitution for most of the nitrogen sites in the trimers. The effects of charge redistribution in the trimers were separated from those of torsional oscillations in several instances including N2 in N2???(HCN)2

ROTATIONAL SPECTRA AND STRUCTURES OF SMALL CLUSTERS CONTAINING THE HCN DIMER - (HCN)2-HF, (HCN)2-HCL, (HCN)2-HCF3, AND (HCN)2-CO2

Microwave rotational spectra have been observed for a number of isotopic species of the (HCN)2???HF, ???HCl, ???HCF3, and ???CO2 trimers. The observations were made with the pulsed nozzle, Fourier transform, Flygare/Balle Mark II spectrometer. The trimers have structures which are composites of the linear (HCN)2 dimer and the HCN???Y dimers, the latter linear for Y=HF and HCl, a symmetric top for Y=HCF3, and T???shaped with C2v symmetry for Y=CO2. The rotational constants for the most abundant species of each trimer are as follows: For Y=HF and HCl, B0 is 699.204 and 467.408 MHz, respectively, and DJ is 162 and 87 Hz; for Y=HCF3, B0 is 305.742 MHz and DJ and DJK are 51 and 471 Hz; for Y=CO2, treated as a symmetric top, (B0+C0)/2 is 452.426 MHz and DJ is 1.057 kHz. Hyperfine interaction constants were determined for several species. The B0???s for each trimer were analyzed by a combination of isotopic substitution and fitting procedures to determine the distances r1 and r2 between the centers of mass (c.m.) of adjacent monomers. The B0???s are relatively insensitive to the position of the central HCN but give r1+r2 accurately. With this limitation, r1 and r2 in the trimers are compared with the corresponding distances in the dimers, which are longer. For Y=HF, HCl, HCF3, and CO2, respectively, the shrinkages found in r1 are 0.069, 0.054, 0.030, and 0.004 ?? and in r2, 0.043, 0.062, 0.042, and 0.052 ??. The shrinkage in r1 and several other properties of the trimers exhibit some correlation with the pseudodiatomic stretching force constant in the HCN???Y dimer

ROTATIONAL SPECTRA AND STRUCTURES OF SMALL CLUSTERS CONTAINING THE HCN DIMER - (HCN)2-AR, A T-SHAPED TRIMER

Microwave rotational spectra have been observed for the four 14N/15N isotopic species of an (HCN)2???Ar trimer with the pulsed nozzle Fourier transform method using the Flygare Mark II spectrometer. Thirteen J???J??? asymmetric top transitions were measured in the 2.5 to 10 GHz region for the parent 14N,14N trimer and nine for each of the other three species. The 14N nuclear quadrupole hyperfine structure was analyzed for the transitions and the interaction constants and line centers determined. The line centers were fitted to obtain ground vibronic state rotational and quartic centrifugal distortion constants. For the 14N/14N trimer these are (in MHz) for A???, B???, and C???: 2013.5993(10), 1759.2756(3), 932.3709(2); and for ??1, ??2, ??aaaa, ??bbbb, and ??cccc: ???0.3017(1), ???0.0660(1), ???0.1700(9), ???0.013???71(1), and ???0.0088(1). The inertial defect is 3.7881 amu?????2. The zero???point vibrationally averaged geometry is planar and T shaped with the structure of the linear HCN dimer remarkably unperturbed by presence of the Ar atom. The substitution N?????????N distance in the cluster is 4.453 ?? compared with 4.461 ?? in the free dimer. The argon is attracted by the ?? electrons of the two ???C???N groups, being equidistant from them. The Ar?????????N(1) and Ar?????????N(2) distances are 3.671 and 4.316 ??, respectively

TUNNELING IN A LINEAR B2H6-HCL DIMER

Rotational spectra have been observed for eight isotopic species of the diborane-HCl complex with a Balle-Flygare, pulsed nozzle, Fourier transform microwave spectrometer. The dimer has a linear, or at most slightly bent B-B...H/DCl equilibrium structure with the H/D end of the HCl attracted symmetrically to a terminal BH2 group of the diborane. Three B2H6-HCl species homonuclear in the boron were observed to tunnel while those with (BBH6)-B-10-B-11 or DCl did not. The tunneling splits each rotational transition into two components of comparable intensity, separated by several MHz depending on J and K. The a-dipole transitions are characteristic of a prolate, very near symmetric top; only K = 0 and +/- 1 transitions were found. Rotational constants are reported for all species. The BBAR, D(J), H, (B-C), and D(JK) constants determined for (B2H6)-B-11-(HCl)-Cl-35 are for the A1 tunneling state 1273.364(1) MHz, 5.56(5) kHz, 1.0(8) Hz, 5.3(2) MHz, and -2.1(3) MHz; for the A2 tunneling state 1273.856(1) MHz, 11.64(7) kHz, 33.1(9) Hz, 5.70(4) MHz, and -3.21(5) MHz. The chlorine hyperfine structure gives the average torsional displacement of the H/DCl from the a axis to be 26.3-degrees for the HCl complexes and 22.5-degrees for the DCl. The torsional displacement of the B2H6 was found by an inertial analysis of the complexes with HCl. It is very anisotropic, being close to 0-degrees in the ethylene plane and about 9-degrees in the BH2B bridging plane. It is suggested that in the tunneling the B2H6 reorients by 180-degrees in the bridging plane, coupled with a gear-like counter rotation of the HCl by 360-degrees. The B...H distance is 2.693 angstrom in the dimers with HCl and the B...D distance is slightly longer, 2.702 angstrom

EXPERIMENTAL SEPARATION OF TORSIONAL AND CHARGE REDISTRIBUTION EFFECTS IN ROTATIONAL SPECTRA OF HCN DIMERS

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 ??1 and ??2 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 ??a between HCN monomer and dimer is caused by charge redistribution. The C-D bond length in the dimer is considered