Carl Chuang

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...

Computer‐based controller and averager for the Balle‐Flygare spectrometer

A computer‐based controller/averager has been built for the Balle‐Flygare pulsed nozzle, Fourier transform, microwave spectrometer. We have integrated gas and microwave pulse control, digitizing and averaging, signal processing, and mirror and frequency control into an IBM PC‐AT, allowing the computer to coordinate all processes in the spectrometer. Multiple free induction decays (FIDs) are recorded for a single gas pulse without delay between digitizing sequences by continuously clocking the FID’s into multiple segments of digitizer memory. The averager fits into one of the AT’s expansion slots and has the unique feature of sharing 16 kbyte of static memory with the CPU. This gives the computer immediate access to the current average since it is already in the computer’s memory. The averaging is very fast so that the nozzle and vacuum pump remain the limiting factors for the repetition rate. Programming features are described. The spectrometer is now easier and faster to run. The increased speed and mult...

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 ...

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.

Rotational spectra and structures of small clusters containing the HCN dimer: (HCN)2–Y with Y=HF, HCl, HCF3, and 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 ...

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 τ1, τ2, τaaaa, τbbbb, and τcccc: −0.3017(1), −0.0660(1), −0.1700(9), −0.013 71(1), and −0.0088(1). The inertial defect is 3.7881 amu A2. 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 ...

Rotational spectra and structures of small clusters containing the HCN dimer: X–(HCN)2 with X=CO, N2, NH3, and H2O

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.)...

Rotational spectra of the ∑ bend states of Ar–H/DCl and the ∑ stretch of Ar–HCl

Rotational transitions of excited van der Waals states of Ar–H/DCl have been observed using the Balle–Flygare pulsed nozzle, Fourier transform, microwave spectrometer. Rotational constants and hyperfine interactions have been determined for the 35Cl/37Cl isotopes of both Ar–HCl and Ar–DCl in the lowest ∑ bending state. The HCl results are in good agreement with previous tunable far‐infrared work [J. Chem. Phys. 89, 1268 (1988)]. For Ar–D35Cl and Ar–D37Cl, the rotational constants B and D and the quadrupole interaction χaa(Cl) were determined to be 1727.9887(1) MHz, 39.505(1) kHz, and −28.246(9) MHz, respectively, and 1686.4892(2) MHz, 39.167(4) kHz, and −22.344(10) MHz. χaa(D) for the Ar–D35Cl was found to be 132(7) kHz. Analysis of the 35Cl/37Cl isotope effects shows that the H(D) in H(D)Cl is pointed predominately away from the Ar. Measurement of the J=0→1 to J=4→5 transitions and their hyperfine structure for the ∑‐stretch state of Ar–H35Cl give the rotational constants B, D, H, L, and the quadrupole i...

Rotational spectrum and structure of the HCN–(CO2)3 tetramer

Microwave rotational transitions have been observed for HCN–(CO2)3, DCN–(CO2)3, H13CN–(CO2)3, HC15N–(CO2)3, HCN–(13CO2)3, HCN–(18OCO)(CO2)2, and HCN–(CO2)(C18O2)2 with the pulsed Fourier transform, Flygare/Balle Mark II spectrometer. A symmetric top spectrum was observed for the parent isotopic species with rotational constants of B0=861.6392(1) MHz, DJ =0.681(5) kHz, and DJK =0.821(12) kHz. The results for isotopic substitution indicate a zero‐point, vibrationally averaged geometry having the C3 symmetry of a cyclic (CO2)3 structure with the HCN along the symmetry axis and the N end closest to the (CO2)3. The C3 symmetry is confirmed by the observation of states limited to K=±3n, with n=0,1,2,..., as predicted for threefold symmetry generated by bosons only. The (CO2)3 has a pinwheel configuration, as in the free trimer, and the three carbons lie in a plane R=2.758 A below the center of mass (c.m.) of the HCN. The C‐C distance in this subunit is 3.797 A which is 0.241 A shorter than that found in the fre...

Rotational spectrum and structure of a linear B2H6–H/DF dimer

Rotational spectra have been observed at 4–18 GHz for several isotopic species of a diborane–hydrogen fluoride complex, using a Flygare, Fourier transform microwave spectrometer with a pulsed supersonic nozzle as the molecular source. The dimer is a near symmetric prolate top (κ=−0.9995); it has a linear BB–H/DF equilibrium structure with the H/D end of the H/DF attracted axially to one of the diborane’s terminal BH2 groups. The B, C, DJ, and DJK rotational constants are: for 11B2H6–HF, 2111.601(1) and 2091.308(1) MHz, and 5.83(3) and 46.1(5) kHz; for 11B2H6–DF, 2092.991(2) and 2072.772(2) MHz, and 5.34(13) and 56.6(11) kHz; and for 10B11BH6–HF, 2176.086(1) and 2154.566(1) MHz, and 6.28(3) and 56.1(5) kHz, respectively. The spectra are insensitive to the value of A, which is assumed to be that of diborane itself (79.6 GHz). The hyperfine structure of the J=0→1 transitions for 11B2H6–HF and 10B11BH6–HF shows that the outer boron nucleus in the dimer has a very small quadrupole coupling constant (‖χaa‖ ≲15 ...