Nuclear quadrupole resonance as a non-destructive testing tool

Nuclear pure quadrupole resonance (NQR) is a resonance technique that provides distinctly different information from that provided by nuclear magnetic resonance (NMR). In NMR the splitting of the energy levels, and therefore the frequency observed, occurs because of the interaction of the nuclear magnetic moment with an external magnetic field. Information about the system under study comes from perturbations on this magnetic interaction. These perturbations lead to a broadening of the line, or to relaxation effects on the interchange of energy between the spins and the lattice, and among the spins. In NQR the primary interaction is between the electric quadrupole moment of a nucleus and the electric field gradient at that nucleus. The field gradient is provided by internal interactions in the sample itself, arising from the chemical bonds, rather than by an external field. Anything that changes the bonding environment, such as tensile stress, will cause shifts in the quadrupole resonance frequency. All nuclei with spin greater than 1/2 have a nuclear quadrupole moment, in addition to their magnetic moment. The nucleus used as an example in this paper is 75As, which has spin 3/2

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

Rotational spectra, structures, and dynamics of small Arm–(H2O)n clusters: The Ar–(H2O)2 trimer

Rotational-tunneling spectra for Ar–(H2O)2 and Ar–(D2O)2 have been observed with the Balle–Flygare Fourier transform microwave spectrometer. The tunneling levels of the trimer appear to correlate with those of the water dimer. The "a" dipole transitions from the A and E+ states of Ar–(H2O)2 and A, B, and E+ states of Ar–(D2O)2 could be fit to a semirigid rotor Watson Hamiltonian. However, only the E+ states give "b" dipole transitions near rigid rotor predictions. The "b" dipole transitions for A and B are rotational-tunneling spectra. For Ar–(D2O)2, these transitions were observed and the donor–acceptor interchange tunneling splitting is determined as 106.3 MHz, compared to about 1100 MHz in the free (D2O)2. From this splitting, the barrier for interchange tunneling is calculated to be 642 cm–1. This splitting for Ar–(H2O)2 is estimated as 4–5 GHz. This and the spin statistical weight of 0 for the B state have made it difficult to observe the "b" dipole rotational tunneling spectra for Ar–(H2O)2. From the rotational constants for (H2 18O) containing trimers, the O–O distance in the trimer is estimated as 2.945 Å. This is significantly (0.035 Å) shorter than the O–O distance reported for water dimer. The Ar is located on the "b" axis of the water dimer. Assuming the water to be a structureless sphere in the trimer, leads to Ar-c.m.(H2O) distance of 3.637 Å, very close to the same value in the Ar–H2O dimer

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 SPECTRA AND STRUCTURES OF THE OC-HCN-HF 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 B0 of 615.574 MHz and DJ of 251 Hz, and for H3N, a symmetric top, a B0 of 1067.161 MHz and DJ and DJK 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 r1, the c.m. to c.m. distance for X???HCN are 0.070 and 0.098 ?? for X=OC and H3N, respectively, and in r2 for HCN???HF, 0.033 and 0.027 ??. The 14N 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

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 ?? below the center of mass (c.m.) of the HCN. The C???C distance in this subunit is 3.797 ?? which is 0.241 ?? shorter than that found in the free (CO2)3 trimer. The individual CO2???s in the (CO2)3 pinwheel are rotated out of the C???C???C plane by ??=???6.9??, as determined from an inertial analysis, with the inner oxygens rotated away from the HCN. The HCN has an average torsional angle of 10.3??, as determined by isotopic substitution, and an observed ??cc value of ???3.891 MHz for the 14 N. The c.m.(HCN) to C distance is 3.525 ??, compared to 3.592 ?? in the HCN???CO2 T???shaped dimer. The isotopic substitution by 18O perturbs the structure of the symmetric top clusters by a remarkable amount, decreasing ?? to ???28.9?? and increasing R and RCC to 2.797 and 3.814 ??, respectively. In the 18O substituted species, the CO2???s are rotated in the C???C???C plane from C3v symmetry by the pinwheel angle ??=???32.5??

Rotational spectra and structures of the Ar3–H2O and Ar3–H2S symmetric tops

Rotational spectra of several isotopomers of Ar3–H2O and Ar3–H2S tetramers were obtained with a Balle–Flygare Fourier transform microwave spectrometer. Both were found to be symmetric tops, the former being an oblate and the latter a prolate one. The rotational constants B, Dj, and DJK were determined to be 1172.1323(1) MHz, 7.199(1) kHz, and –5.545(2) kHz for the H2O and 819.0385(1) MHz, 3.346(1) kHz, and +3.145(2) kHz for the H2S containing tetramer. Substitution analysis with the rotational constants of various isotopomers led to an Ar–Ar distance of 3.848 (3.865) Å and an Ar-c.m.(H2X) distance of 3.675 (4.112) Å for H2O (H2S) complexes. The angle between the C2 axis of the H2X and the C3 axis of the tetramer is estimated to be 74° for H2O and 13° for the H2S complex. No evidence for any excited tunneling/internal rotor states was found for either of the tetramer. MMC calculations show that the equilibrium geometry has the H2X positioned above the plane of the Ar3 with both the protons pointing towards one Ar each. The barrier for the "pseudorotation" in which the protons hop between the argons is determined to be about 6 (8) cm–1 only for H2O (H2S) making the H2X moiety very mobile and effectively making both the tetramers symmetric tops. Rigid body diffusion quantum Monte Carlo (RBDQMC) calculations with the MMC potential have been carried out for vibrational analysis