Deuterium Quadrupole Coupling Constants Research Articles

Conformations and Low-Frequency Intramolecular Motions of 1-Butanol, 1-Butanethiol, Iso-butanol, and Iso-butanethiol Investigated by Fourier Transform Microwave Spectroscopy Combined with Quantum Chemical Calculations.

The rotational spectra of 1-butanol (1-BuOH), 1-butanethiol (1-BuSH), 2-methyl-1-propanol (iso-BuOH), and 2-methyl-1-propanethiol (iso-BuSH) were measured by Fourier transform microwave spectroscopy in the frequency region from 3.7 up to 25 GHz. The observed spectral lines were assigned by observation of the deuterium substitution effect and by ab initio or density functional theory calculations at the levels of MP2/6-311++G(d,p) or B3LYP and cam-B3LYP, respectively. For 1-BuOH and 1-BuSH, seven of the 14 conformations, anticipated to exist as stable, were detected, whereas four and three among the five possible conformations were identified for iso-BuOH and iso-BuSH, respectively. We further found that, of the seven conformers of 1-BuOH, five were trans and two gauche, with respect to the internal rotation axis: the C2-C3 bond, while three of iso-BuOH existed in gauche and one in trans. The most stable conformer of the two BuOH molecules was trans with respect to the C-O bond, while all the sulfur analogues were gauche to the C-S axis. The rare isotopomers examined included 13C and OD of 1-BuOH and OD of iso-BuOH, 34S, 13C, and SD of the two sulfur molecules, and the rotational constants obtained on these isotopomers were employed in the molecular structure derivation. The potential barrier to CH3 internal rotation and the deuterium quadrupole coupling constant, where available, were also derived from the spectral analysis, and the molecular parameters thus obtained were compared with those derived using quantum-chemical calculations; the values derived using cam-B3LYP/6-311++G(d,p) were in better agreement with the observed than those derived using MP2/6-311++G(d,p) and B3LYP/6-311++G(d,p). The TTg form of 1-BuOH and of 1-BuSH and the Tg form of iso-BuSH exhibited additional spectral splittings, which were interpreted as caused by the OH or SH group tunneling between the symmetric and antisymmetric states. Some of the J = 8 rotational levels of 1-BuSH happened to be near-degenerate with others, and the splittings in them caused by mutual repulsion could be precisely determined by the observation of the transitions involving those split levels. Such splittings were determined for 1-BuSH, 1-BuSD, and iso-BuSH to be 1694.1731 (22), 56.3174 (16), and 6.4678 (14) MHz, respectively. A natural bond orbital analysis was performed to show that the most stable conformation of the primary and secondary alcohols is Gt because of the charge transfer from the lone-pair electron of the oxygen atom to the antibonding orbital of the C-H bond in 1-BuOH, whereas in iso-BuOH, the charge transfer to the antibonding orbital of the C1-C2 bond.

Solution and Solid‐State Structure of the First NHC‐Substituted Rhenium Heptahydrides

The structure of three new bis(N‐heterocyclic carbene) heptahydride rhenium complexes ([ReH7(NHC)2] with NHC= IMes (2b) (1,3‐bis(2,4,6‐trimethyl phenyl) imidazol‐2‐ylidene), IMesCl2 (4b) (4,5‐dichloro‐1,3‐dihydro‐1,3‐bis(2,4,6‐trimethyl phenyl)‐2H‐imidazol‐2‐ylidene) BIMes (5b) (1,3‐Bis(2,4,6‐trimethylphenyl)‐benzo imidazole ylidene), and of the known complex [ReH7(PPh3)2] (1) (bis(triphenyl phosphine)‐rhenium heptahydride), and the one of their labelled counterparts 2c‐d7, 4c‐d7, and 5c‐d7, were established in solution. The compounds exchanged with HD (2 bar) at 60 °C in deuterated THF to produce statistically distributed isotopomers. This exchange pointed to the intermediacy of H2 complexes and dynamic systems with fast equilibration processes of all hydride positions. T1min measurements were run on 2b, 4b, 5b and 1 in deuterated methylcyclohexane: 2b T1min = 144 ms, 5b T1min = 127 ms, both at 203 K. For 4b two T1min values were obtained corresponding to the 1H‐NMR signals of the hydrides: T1min = 119 ms and T1min = 128 ms both at 203 K. The T1min values determined are typical of terminal rhenium polyhydrides and for 1 was reported a comparably short value of T1min (70 ms, [D8]toluene, at –73 °C), typical of complexes that contain dihydrogen ligands. D‐NMR T1min measurements were also run for the fully deuterated compounds 2c‐d7, 4c‐d7, 5c‐d7 and of 1c‐d7 allowing to calculate the ionicities of the metal hydride bonds. The Re‐D ionicity values were obtained from the Deuterium Quadrupole Coupling Constant (DQCC) determinations. They were in the range of 0.61–0.71 for the fully labelled complexes: 2c‐d7: T1min = 15 ms, i = 61 %; 4c‐d7: (A) T1min = 20 ms, i = 67 %, (B) T1min = 41 ms, i = 77 %; 5c‐d7: T1min = 24 ms, i= 70 %; and 1c‐d7: T1min = 26 ms, i = 71 %. Single crystal X‐ray diffraction studies on 4b enabled us to refine the hydride positions with only restraints in the bond lengths and thermal parameters. The X‐ray solid structure resembles a pseudo‐capped square antiprismatic geometry.

Supersonic jet cooled rotational spectrum of 2,4-difluorophenol

The microwave spectrum of the cis form of aromatic 2,4-difluorophenol (DFP) has been recorded and analyzed in the frequency range of 5–25GHz using a pulsed-jet Fourier transform microwave spectrometer. Rotational transitions were measured for the parent and all unique single 13C substituted isotopologues and 18O in natural abundance and on enriched deuterium species on the hydroxyl group. The rotational (MHz), centrifugal distortion (kHz), and quadrupole coupling constants (MHz) in deuterium species were determined. The rotational constants for the parent species are obtained as A=3125.04158(43)MHz, B=1290.154481(54)MHz, C=913.197424(36)MHz, DJ = 0.020899(162)kHz, DK=0.9456(100)kHz, DJK=0.09273(65)kHz, d1=−0.00794(14)kHz, d2=−0.002356(93)kHz and for the deuterated species A=3125.38579(44)MHz, B=1261.749784(48)MHz, C=898.927184(27)MHz, DJ = 0.02096(19)kHz, DK=0.379(74)kHz, DJK=0.0880(11)kHz, d1=−0.00691(11)kHz, d2=−0.00183(11)kHz. The deuterium quadrupole coupling constants are χaa=−0.0109(33)MHz, and (χbb−χcc)=0.2985(59)MHz. The rs substitution structure was determined using the measured rotational constants of the isotopologues, a nonlinear least squares fit was performed to obtain the best fit gas phase r0 effective structure.Supporting ab initio (MP2) and density functional calculations provided consistent values for the rotational parameters, and molecular structure.

Nuclear Quadrupole Resonance Study of Proton and Deuteron Migration in Short Strong Hydrogen Bonds Formed in Molecular Complex 3,5-Dinitrobenzoic Acid–Nicotinic Acid and in Deuterated 3,5-Pyridinedicarboxylic Acid

Temperature dependences of the 14N nuclear quadrupole resonance (NQR) frequencies have been measured in molecular complex 3,5-dinitrobenzoic acid–nicotinic acid (35DBNA) and in deuterated 3,5-pyridinedicarboxylic acid (DPDA). In deuterated DPDA, the temperature dependences of the deuterium quadrupole coupling constants and asymmetry parameters η have also been measured. In 35DBNA, the magnitude of the quadrupole coupling constant of the pyridine nitrogen agrees with the experimentally observed proton transfer. No proton exchange (O–H···N ↔ O–···H–N+) is observed. The temperature dependence of the 14N quadrupole coupling constant is analyzed in a model of a resonance hybrid of two extreme electron configurations of the molecules. The energy difference of the two extreme electron configurations is determined. In the same model, we analyzed previously published 14N NQR data of the NHO hydrogen bond in nondeuterated 3,5-pyridinedicarboxylic acid. The 14N NQR data in DPDA show the presence of a quasi-continuou...

Measurements of deuterium quadrupole coupling in propiolic acid and fluorobenzenes using pulsed-beam Fourier transform microwave spectrometers.

The pure rotational spectra of deuterated propiolic acids (HCCCOOD and DCCCOOH), 1-fluorobenzene (4-d1), and 1,2-difluorobenzene (4-d1) in their ground states have been measured using two Fourier transform microwave (FTMW) spectrometers at the University of Arizona. For 1-fluorobenzene (4-d1), nine hyperfine lines of three different ΔJ = 0 and 1 transitions were measured to check the synthesis method and resolution. For 1,2-difluorobenzene (4-d1), we obtained 44 hyperfine transitions from 1 to 12 GHz, including 14 different ΔJ = 0, 1 transitions. Deuterium quadrupole coupling constants along the three principal inertia axes were well determined. For deuterated propiolic acids, 37 hyperfine lines of Pro-OD and 59 hyperfine lines of Pro-CD, covering 11 and 12 different ΔJ = - 1, 0, 1 transitions, respectively, were obtained from 5 to 16 GHz. Deuterium quadrupole coupling constants along the three inertia axes were well resolved for Pro-OD. For Pro-CD, only eQq(aa) was determined due to the near coincidence of the CD bond and the least principal inertia axis. Some measurements were made using a newer FTMW spectrometer employing multiple free induction decays as well as background subtraction. For 1-fluorobenzene (4-d1) and 1,2-difluorobenzene (4-d1), a very large-cavity (1.2 m mirror dia.) spectrometer yielded very high resolution (2 kHz) spectra.

A HIGH-RESOLUTION ISOTOPIC STUDY OF THE ROTATIONAL SPECTRUM OF c -C 3 H 2

The rotational spectra of the normal and seven isotopic species of cyclopropenylidene c-C{sub 3}H{sub 2} have been measured at high spectral resolution by Fourier transform microwave spectroscopy of a supersonic molecular beam between 10 and 43 GHz. Deuterium quadrupole coupling and carbon-13 spin-rotation hyperfine constants were determined in addition to the rotational constants. Quartic and sextic centrifugal distortion constants derived from 28 lines between 150 and 316 GHz of the doubly deuterated species c-C{sub 3}D{sub 2} allow the rotational spectrum to be calculated to 0.5 km s{sup -1} or better in equivalent radial velocity up to 500 GHz. Spectroscopic constants determined from four centimeter-wave and 19 millimeter-wave lines of the normal species c-C{sub 3}H{sub 2}, including 15 with sharp Lamb-dips, allow prediction of the most important astronomical transitions (i.e., those with {Delta}J = 1 and K{sub a} {<=} 3) to 0.05 km s{sup -1} or better at 500 GHz. The doubly deuterated species is a good candidate for detection in cold dark clouds, because deuterium fractionation is high in c-C{sub 3}H{sub 2} and lines of C{sub 3}HD are fairly intense in these sources. An accurate empirical equilibrium structure of c-C{sub 3}H{sub 2}, derived from the experimental rotational constants of normalmore » and isotopic c-C{sub 3}H{sub 2}, corrected for zero-point vibrational effects, is compared with previously reported structures.« less

Nuclear magnetic resonance parameters in water dimer

The changes in the computed nuclear magnetic resonance (NMR) parameters of the water dimer with respect to their monomer values were monitored as the geometry of the dimer was systematically varied. Nuclear magnetic shielding constants, shielding tensor anisotropies, nuclear quadrupole coupling constants and spin–spin coupling constants for the hydrogen bond donor and acceptor molecules were calculated at hybrid density-functional theory level. The dimer geometry was specified through the intermolecular oxygen–oxygen distance ROO and the hydrogen bond angle α. A grid of 120 geometries was selected by systematically varying these two parameters. The other geometrical parameters of the dimer were allowed to relax, keeping the two parameters fixed. As the dimer geometry was varied, all NMR parameters were observed to be smoothly behaving. Characteristic changes as a function of the intermolecular geometry were observed. These include, besides the well-known deshielding of the donor hydrogen shielding constant, also influences on the donor deuterium quadrupole coupling constant, as well as the shielding anisotropy of the donor and acceptor oxygens. We discuss the contributions to the total dimerisation effect from, on the one hand, the dominant direct interaction effect at a fixed geometry and, on the other hand, from the quantitatively relevant indirect, geometric effect. A fundamental ambiguity of this partitioning is demonstrated. By forging the general, smooth trends in all the studied NMR parameters into a specific geometric definition, we find our data to be in agreement with the widely used distance criterion for hydrogen bonding in water, ROO ≤ 3.5 A.

High-resolution rotational spectroscopy of NNOH +, DCS +, [formula omitted], Ar ⋯ DCO +, and [formula omitted

The rotational spectra of the protonated molecules NNOH + and DCS +, and the ion complexes Ar ⋯ D 3 + , Ar ⋯ DCO +, and Ar ⋯ HNN + have been detected by Fourier transform microwave spectroscopy at high spectral resolution, in which the supersonic molecular beam expands along the axis of the Farby–Perot cavity. These new measurements either better resolve or reveal additional line structure not observed in the previous lower resolution studies. Precise nitrogen or deuterium quadrupole coupling constants have been determined for the first time for both ions and Ar ⋯ DCO +; a tunneling splitting of 72 ± 5 kHz has been measured for the 1 0 , 1 → 0 0 , 0 transition of Ar ⋯ D 3 + .

Isofulminic acid, HONC: Ab initio theory and microwave spectroscopy

Isofulminic acid, HONC, the most energetic stable isomer of isocyanic acid HNCO, higher in energy by 84 kcal/mol, has been detected spectroscopically by rotational spectroscopy supported by coupled cluster electronic structure calculations. The fundamental rotational transitions of the normal, carbon-13, oxygen-18, and deuterium isotopic species have been detected in the centimeter band in a molecular beam by Fourier transform microwave spectroscopy, and rotational constants and nitrogen and deuterium quadrupole coupling constants have been derived. The measured constants agree well with those predicted by ab initio calculations. A number of other electronic and spectroscopic parameters of isofulminic acid, including the dipole moment, vibrational frequencies, infrared intensities, and centrifugal distortion constants have been calculated at a high level of theory. Isofulminic acid is a good candidate for astronomical detection with radio telescopes because it is highly polar and its more stable isomers (HNCO, HOCN, and HCNO) have all been identified in space.

2H Solid-State NMR of Ruthenium Complexes

The (2)H solid-state NMR spectra of the transition metal complexes Tp*RuD(THT)(2) (1a), Tp*RuD(D(2))(THT) (1b), Tp*RuD(D(2))(2) (1c), Cp*RuD(3)(PPh(3)) (2) and RuD(2)(eta(2)-D(2))(2)(PCy(3))(2) (3) have been measured in a wide temperature range. These compounds were chosen as potential model systems for hydrogen surface species in Ru-nanoparticles. The deuterium quadrupolar coupling constants Q(cc) and asymmetry parameters were extracted by (2)H NMR line-shape analysis. The Q(cc) values of the deuterons bound to the metal vary between 13 kHz and 76 kHz. In addition all spectra show that some of the deuterium is incorporated into carbon positions exhibiting quadrupolar coupling constants in the range of 134 kHz to 192 kHz. The room temperature spectra contain an additional weak very narrow line which was assigned to deuterons exhibiting a high mobility. These deuterons are attributed to crystallographic impurity and partially to D(2) molecules which lost by the complexes. The temperature where their motion is quenched and the types of these motions depend on the chemical structure. We propose to use the values of the quadrupolar coupling constants measured in order to characterize different hydrogen species on the surface of Ru-nanoparticles.

Sub-Doppler millimetre-wave spectroscopy of DBS and HBS: accurate values of nuclear electric and magnetic hyperfine structure constants

The unstable thioborine molecule and its deuterated variant have been produced by a high-temperature reaction between hydrogen sulfide and crystalline boron at 1100 degrees C in a flow system. Five rotational transitions from J = 2 <-- 1, to J = 6 <-- 5 have been recorded with sub-Doppler resolution for the vibrational ground state of H10/11BS and D10/11BS using the Lamb-dip technique. The hyperfine structure due to the electric quadrupole interaction of deuterium nucleus has been resolved yielding the first experimental determination of the deuterium quadrupole coupling constant in thioborine, which is 0.1403(75) MHz in D11 BS and 0.1360(38) MHz in D10BS. Fairly accurate values of 10/11B spin-rotation coupling constants and of the hydrogen-boron spin-spin coupling constants have also been determined. Additionally, the hyperfine structure of the rotational lines for the nu2 = 1 excited state has been investigated, thus obtaining information on the asymmetry of the electric field gradient at the B nucleus in the bending state.

Dynamics of Dilute Water in Carbon Tetrachloride

A dilute solution of water in a hydrophobic solvent, such as carbon tetrachloride (CCl4), presents an opportunity to study the rotational properties of water without the complicating effects of hydrogen bonds. We report here the results of theoretical, experimental, and semiempirical studies of a 0.03 mole percent solution of water in CCl4. It is shown that for this solution there are negligible water-water interactions or water-CCl4 interactions; theoretical and experimental values for proton NMR chemical shifts (deltaH) are used to confirm the minimal interactions between water and the CCl4. Calculated ab initio values and semiempirical values for oxygen-17 and deuterium quadrupole coupling constants (chi) of water/CCl4 clusters are reported. Experimental values for the 17O, 2H, and 1H NMR spin-lattice relaxation times, T1, of 0.03 mole percent water in dilute CCl4 solution at 291 K are 94+/-3 ms, 7.0+/-0.2 s, and 12.6+/-0.4 s, respectively. These T1 values for bulk water are also referenced. "Experimental" values for the quadrupole coupling constants and relaxation times are used to obtain accurate, experimental values for the rotational correlation times for two orthogonal vectors in the water molecule. The average correlation time, tauc, for the position vector of 17O (orthogonal to the plane of the molecule) in monomer water, H2(17)O, is 91 fs. The average value for the deuterium correlation time for the deuterium vector in 2H2O is 104 fs; this vector is along the OD bond. These values indicate that the motion of monomer water in CCl4 is anisotropic. At 291 K, the oxygen rotational correlation time in bulk 2H2(17)O is 2.4 ps, the deuterium rotational correlation time in the same molecule is 3.25 ps. (Ropp, J.; Lawrence, C.; Farrar, T. C.; Skinner, J. L. J. Am. Chem. Soc. 2001, 123, 8047.) These values are a factor of about 20 longer than the tauc value for dilute monomer water in CCl4.

Trans-W(Cmesityl)(dmpe)2H: Revealing a Highly Polar W−H Bond and H-Mobility in Liquid and Solid State

The isotopomeric complexes trans-W(Cmesityl)[(C(H,D)3)2PCH2CH2P(C(H,D)3)2]2(H,D) 1-4 were prepared. 2 (W(Cmesityl)(dmpe)2D) was used to study the Deuterium Quadrupole Coupling Constant (DQCC) and the ionicity of the W-D bond (DQCC=34.1 kHz; ionicity 85%). 1 (W(Cmesityl)(dmpe)2H) shows several dynamic exchange processes in solution, such as HW/HW, HW/ortho-Memesityl, and HW/H2 exchanges observed by NMR in combination with deuterium labeling studies and double label crossover experiments. Except for the HW/H2, these reactions comprise elementary steps, which also appear along the isomerization pathway of 1 into (2,3,5-trimethylphenylcarbyne)(dmpe)2WH (5) at 60 degrees C. 5 was characterized by an X-ray diffraction study. In the solid state only an HW/Mep exchange process prevails appearing at higher temperatures, which was identified by NMR and by Quasielastic Neutron Scattering. The latter also provided an activation barrier of 5 kcal/mol and a "jump width" for the moving H nucleus in agreement with the HW...Mep distance of the X-ray diffraction study of 1.

A quantitative measure of the ionicity of selected iron hydride species by examination of the deuterium quadrupole coupling constant (DQCC)

The ionicity of the metal hydride bonds of a series of iron hydrides supported by multidentate phosphine donors has been determined by means of DQCC measurements of the isotopically labelled hydrides in concentrated solution in toluene.

Experimental and quantum-chemical determination of the2H quadrupole coupling tensor in deuterated benzenes

Deuterium Quadrupole Coupling Constant (DQCC) in benzene was determined both experimentally by Nuclear Magnetic Resonance spectroscopy in Liquid Crystalline solutions (LC NMR) and theoretically by ab initio electronic structure calculations. DQCCs were measured for benzene-d(1) and 1,3,5-benzene-d(3) using several different liquid crystalline solvents and taking vibrational and deformational corrections into account in the analysis of experimental dipolar couplings, used to determine the orientational order parameter of the dissolved benzene. The experimental DQCC results for the isotopomers benzene-d(1) and 1,3,5-benzene-d(3) are found to be 187.7 kHz and 187.3 kHz, respectively, which are essentially equal within the experimental accuracy (+/-0.4 kHz). Theoretical results were obtained at different C-D bond lengths, and by applying corrections for electron correlation and rovibrational motion on top of large-basis-set Hartree-Fock results. The computations give a consistent DQCC of ca. 189 kHz for three different isotopomers; benzene-d(1), 1,3,5-benzene-d(3), and benzene-d(6), revealing that isotope effects are not detectable within the present experimental accuracy. Calculations carried out using a continuum solvation model to account for intermolecular interaction effects result in very small changes as compared to the data obtained in vacuo. The comparison of theoretical and experimental results points out the selection of the underlying molecular geometry as the most likely source of the remaining discrepancy of less than 2 kHz. Such an agreement between the calculated and the experimental DQCC results can only be achieved if rovibrational effects are considered on one hand in the experimental direct dipolar coupling data, and on the other hand in the theoretical property calculation, as is done presently.

Practical NMR relaxation for chemists

Preface. 1. How and Why Nuclei Relax. 1.1. Nucleus in the magnetic field. 1.2. Spin--lattice and spin--spin nuclear relaxation. 1.2.1. Macroscopic magnetization: relaxation times T1 and T2. 1.3. Molecular motions as reason of nuclear relaxation. 1.3.1. Correlation times and activation energies of Molecular Motions. 1.3.2. Isotropic and anisotropic molecular motions. 1.4. Bibliography for Chapter 1. 2. How to Measure the NMR Relaxation Times. 2.1. Exponential and non--exponential nuclear relaxation. 2.2. Measurements of spin--lattice relaxation times. 2.3. Measurements of selective and bi--selective T1 times. 2.4. Determinations of T1( and T2 times. 2.5. Preparation of samples for relaxation experiments. 2.6. Bibliography to Chapter 2. 3. Errors in Determinations of Relaxation Times. 3.1. Instrumental errors. 3.2. Incorrect parameters for T1, T2 measurements and T1, T2 calculations. 3.3 Coupled nuclear relaxation. 3.4. Chemical exchanges. 3.5. Bibliography to Chapter 3. 4. NMR Relaxation by Dipole--Dipole and Quadrupole Interactions. 4.1. The intramolecular dipole--dipole relaxation: homo-- and hetero--nuclear dipolar coupling and the spectral density function. 4.2. Haw to reveal the presence of the dipolar mechanisms. 4.2.1. NOE as a test for dipole--dipole nuclear relaxation. 4.2.2. Evaluations of the dipolar contributions from selective and non--selective T1 times. 4.3. Intermolecular dipole--dipole interactions. 4.4. Electric field gradients at quadrupolar nuclei. 4.5. Nuclear quadrupole coupling constant as a measure of the electric field gradient. 4.6. Quadrupole relaxation. 4.7. Bibliography to Chapter 4. 5. Relaxation by Chemical Shift Anisotropy, Spin--Rotation Relaxation, Scalar Relaxation of the Second Kind and Cross--Mechanisms. 5.1. Relaxation by chemical shift anisotropy. 5.2. Spin--rotation relaxation. 5.3. Interference mechanisms of nuclear relaxation. 5.4. The scalar relaxation of the second kind. 5.5 Bibliography to Chapter 5. 6. Nuclear Relaxation in Molecular Systems with Anisotropic Motions. 6.1. Spin--lattice nuclear relaxation in ellipsoid molecules: Temperature dependences of T1times. 6.2. How to reveal anisotropic molecular motions in solutions. 6.3. Nuclear relaxation in the presence of correlation time distributions. 6.4. Bibliography to Chapter 6. 7. 1H T1 Relaxation Diagnostics in Solutions. 7.1. Revealing weak intermolecular interactions by T1 time measurements in solutions. 7.2. T1 studies of exchanges in simple molecular systems. 7.3. Structural 1H T1 criterion. 7.4. Partially--relaxed NMR spectra. 7.5. Bibliography to Chapter 7. 8. Internuclear Distances from the 1H T1 Relaxation Measurements in Solutions. 8.1. X...H distances: metal -- hydride bond lengths. 8.1.1. How to determine metal--hydride bond lengths by standard 1H T1 measurements. 8.1.2. Metal--hydride bond lengths by 1H T1sel and 1H T1min times measurements. 8.2. Proton--proton distances by standard T1 measurements. 8.3. H--H distances from T1sel / T1bis measurements. 8.4. H--H distances in intermediates. 8.5. Analyzing the errors in 1H T1 determinations of internuclear distances. 8.6. Bibliography to Chapter 8. 9. Deuterium Quadrupole Coupling Constants from 2H T1 Relaxation Measurements in Solutions. 9.1. How to determine DQCC values. 9.2. DQCC values from the 2H T1 times measurements in solutions (fast motional regime). 9.3. DQCC values via 2H T1min measurements in solutions. 9.4. Errors in DQCC determinations. 9. 5. Bibliography to Chapter 9. 10. Spin--Lattice 1H and 2H Relaxation in Mobile Groups. 10.1. 1H T1 times and H--H distances in the presence of fast vibrations and librations. 10.2. 1H T1 times and H--H distances in the presence of fast rotational diffusion. 10.3. The spectral density function for high--amplitude librations. 10.4. 900--jumps in four--fold potential. 10. 5. Deuterium spin--lattice NMR relaxation in mobile molecular fragments. 10.6. Bibliography to Chapter 10. 11. Relaxation of Nuclei Other Than 1H and 2H) and Specific Relaxation Experiments. 11.1. Chemical shift anisotropies and nuclear quadrupole coupling constants from T1 times of heavy nuclei in solutions. 11.2. Multinuclear relaxation approaches to complexation, association and H--bonding. 11.3. Na relaxation in solutions of complex molecular systems. 11. 4. Character of molecular motions from 17O and 2H T1 relaxation in solutions. 11.5. 2D T1 and T1( NMR experiments. 11.6. Chemical exchanges in complex molecular systems from 15N nuclear relaxation in solutions. 11.7. R1/R2 method. 11.8. Cross--correlation relaxation rates and structure of complex molecular systems in solutions. 11.9. Variable -- field relaxation experiments. 11.10. Bibliography to Chapter 11. 12. Paramagnetic NMR Relaxation. 12.1. Theoretical basics of the paramagnetic relaxation enhancement. 12.2. Paramagnetic relaxation rate enhancements in the presence of chemical exchanges. 12.3. Structural applications of paramagnetic relaxation rate enhancements. 12.4. Kinetics of ligand exchanges via paramagnetic relaxation rate enhancements. 12.5. Longitudinal electron relaxation time in paramagnetic centers from variable--high field NMR experiments. Bibliography. Concluding Remarks. Index.

Deuterium Quadrupolar Coupling Constants of Deuteride Bridging Ligands: A Study on Rhenium Hydrido Carbonyl Clusters

Deuterium variable-temperature T1 measurements have been performed for a series of rhenium carbonyl clusters containing doubly and triply bridging deuterides, namely Re2(μ-D)2(CO)8 (1), Re3(μ-D)3(CO)12 (2), Re4(μ3-D)4(CO)12 (3), [NEt4][Re3(μ-D)4(CO)10] (4), [NEt4][Re3(μ-D)3(μ-Cl)(CO)10] (5), and [NEt4][Re3(μ-D)4(CO)9] (6). The deuterium quadrupolar coupling constants (DQCC) of the deuteride ligands have been evaluated for compounds 1−5, while the fluxionality of 6 hampered such a determination. The experimental DQCC of compound 3 (21.8 kHz) was, until now, the smallest found for organometallic compounds. Overall the DQCC values span a 4-fold range, and their fine-tuning depends on the bridging mode of the deuteride ligands (μ3 vs μ), the local unsaturation degree of the Re atoms bridged by the deuteride, and the presence of other bridging ligands. The experimental DQCC's of rhenium derivatives and those of bridging deuterides in Cr, W, and Os clusters linearly correlate with the respective metal−hydrogen ...

Molecular Structure of o-Benzyne from Microwave Measurements

The o-benzyne molecule has been known for many years to be an important, but short-lived, reaction intermediate in substitution reactions and more recently in cyclization reactions of enediynes. Although there has been widespread interest in this transient molecule, previous experimental structural data were very limited. In the present work, rotational transitions for o-benzyne were measured with a pulsed-beam, Fourier transform microwave spectrometer for all unique, singly substituted 13C and single-D isotopomers. The o-benzyne was efficiently produced by flowing a dilute mixture of isotopically enriched benzene in neon through a pulsed-DC discharge beam source. The new data, combined with previous data for the normal isotopomer, provide a complete set of structural parameters for this molecule. The rs substitution coordinates and the coordinates from a least-squares fit are reported and are in good agreement. When using the least-squares fit to obtain structural parameters, correction terms arising from harmonic terms in the vibrational averaging were subtracted from the measured rotational constants to obtain a better representation of the planar equilibrium structure. Further improvements in the fits were obtained by applying small, mass-dependent adjustments to the atom coordinates. Structural parameters obtained from the fit to these modified rotational constants are an acetylenic C1⋮C2 bond length of 1.264(3) Å, and the other bond lengths C2−C3 = 1.390(3) Å, C3−C4 = 1.403(3) Å, C4−C5 = 1.404(3) Å, C3−H1 = 1.095(9) Å, and C4−H2 = 1.099(4) Å. The C1⋮C2 bond is only 0.057 Å longer than the free acetylene bond. The other C−C bond lengths are within 0.01 Å of those of benzene C−C bonds. New spectral data for the single-D isotopomers were used to obtain better values for the deuterium quadrupole coupling. Bond-axis deuterium quadrupole coupling constants are eQqzz(D1) = 188(2) kHz, and eQqzz(D2) = 185(10) kHz, which agree well with the value for benzene-D1. The new structural parameters are compared here with theoretical parameters and with an NMR measurement of the C1−C2 bond length.

A study of long range dipolar coupling constants in 4′-pentyloxy-4-biphenylcarbonitrile (5OCB)

It was shown previously that long range 2H-13 C dipolar coupling constants in liquid crystals could be obtained from the proton-decoupled 13C NMR spectrum of monodeuterated compounds. This approach has now been applied to the study of 4′-(1-deuteropentyloxy)-4-biphenylcarbonitrile (5OCB-d1). The 113C spectrum of this compound gives nine 2H- 13C dipolar coupling constants for carbon ranging from one to six bonds away from the dueterium nucleus. The 2 H NMR and 5OCB-d1 gives the deuterium quadrupole coupling constant and the 1H- 2H dipolar coupling constant for the α -CHD group. In addition, eleven 2-bond and two 3-bond 1H- 13C dipolar coupling constants have been obtained for 4′-pentyloxy-4-biphenylcarbonitrile (non-deuterated 5OCB) by using the 2D 13C method of proton-encoded local field spectroscopy.

Deuterium NMR and Ab Initio Studies of Strongly Hydrogen-Bonded Molecules

Two newly synthesized alkylated cyanomalonaldehyde derivatives possess very short intramolecular hydrogen bonds, which are studied by deuterium NMR. Both the dimethyl and di- tert-butyl derivatives have small deuterium quadrupole coupling constants and large asymmetry parameters. In addition, the di- tert-butyl compound, which has one of the shortest O– D…O hydrogen bonds yet measured, has a quadrupole coupling which anomalously increases with temperature. The significance of these phenomena is explored using a theoretical model which employs vibrationally averaged ab initio electric field gradient tensors calculated with large basis sets and electron correlation via the multiconfigurational self-consistent field method.