Re-examination of the hyperfine structure of 14NH2 (original) (raw)
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The Journal of Physical Chemistry A, 2003
Isotropic hyperfine coupling constants (iHFCCs) can be easily measured by electron spin resonance spectroscopy in solution, but they have proven difficult to calculate from first principles. We test the performance of the newly developed (aug-)cc-pVXZ-t5s basis sets for hydrogen with Dunning's (aug-)cc-pVXZ and -pCVXZ basis sets for non-hydrogen atoms. Correlation is included by CCSD and CCSD(T) using UHF and ROHF references. A two-point extrapolation of cc-pVDZ:cc-pVDZ-t5s-a5 and cc-pVTZ:cc-pVTZ-t5s-a6 hydrogen iHFCCs is found to be very useful. Diffuse functions have nearly no influence on extrapolated iHFCCs. We also explore the dependence of the calculated iHFCCs on the level of theory used in optimizing the geometries. For this purpose, we optimized geometries up to the UHF-CCSD/cc-pCVQZ and UHF-CCSDT/cc-pCVTZ levels and extrapolated to the "complete basis set" limit. The calculated iHFCCs are compared to reference values, which are experimental numbers corrected for solvent and the most important vibrational effects. Our test molecules are the CH 3 • , C 2 H 3 • , and H 2 CN • radicals. At the highest level of theory, the largest deviations from the reference values are smaller than 3.5 G and 6%. The rms errors are below 2.1 G and 4%. The cc-pVXZ:cc-pVXZ-t5s basis set combinations perform better than the EPR-n and the Chipman [631|41] basis set. All of them are better than similarly sized basis sets that were not developed for iHFCCs. The calculated iHFCCs are influenced most strongly by the choice of basis set, the perturbative inclusion of connected triple excitations, and the choice of reference wave function and the level of theory in geometry optimization. Core correlation is necessary for the computation of iHFCCs for non-hydrogen atoms but has very little influence on the iHFCCs of hydrogen atoms. A good compromise between the cost and accuracy of hydrogen iHFCCs seems to be reached by two-point extrapolated ROHF-CCSD(T)-fc iHFCCs at UHF-MBPT(2)-fc/cc-pVTZ geometries. ROHF-MBPT(2)-fc or UHF-CCSD-fc/cc-pVTZ geometries are necessary when single excitations are not negligible.
Laboratory Measurements of the Hyperfine Structure of H 14 N 12 C and D 14 N 12 C
The Astrophysical Journal, 2006
The nuclear quadrupole hyperfine structure of H 14 N 12 C and D 14 N 12 C has been resolved in the laboratory for the first time using millimeter-wave absorption spectroscopy. The transient species were produced in a pulsed DC discharge nozzle, and Doppler broadening effects were minimized by propagating the millimeter waves coaxially with the supersonic molecular beam. New rest frequencies for the , , and J p 1-0 J p 2-1 J p 3-2 rotational transitions of the ground vibrational state were determined. The nuclear quadrupole coupling constants derived from the spectra are kHz for H 14 N 12 C and kHz and (eQq) p 264.5 ע 4.6 (eQq) p 294.7 ע 13.
Ro-vibrational averaging of the isotropic hyperfine coupling constant for the methyl radical
The Journal of chemical physics, 2015
We present the first variational calculation of the isotropic hyperfine coupling constant of the carbon-13 atom in the CH3 radical for temperatures T = 0, 96, and 300 K. It is based on a newly calculated high level ab initio potential energy surface and hyperfine coupling constant surface of CH3 in the ground electronic state. The ro-vibrational energy levels, expectation values for the coupling constant, and its temperature dependence were calculated variationally by using the methods implemented in the computer program TROVE. Vibrational energies and vibrational and temperature effects for coupling constant are found to be in very good agreement with the available experimental data. We found, in agreement with previous studies, that the vibrational effects constitute about 44% of the constant's equilibrium value, originating mainly from the large amplitude out-of-plane bending motion and that the temperature effects play a minor role.
The Journal of Chemical Physics, 1992
The hyperfine coupling constants (hfcc) Aiso and Aij are calculated for the atoms of NH2 in its two lowest-lying electronic states at various molecular geometries by means of the ab initio multireference configuration interaction method. The vibronically averaged values of the hfccs for the K=0 and 1 levels in 14N 1H2 in the energy range up to 20 000 cm−1 are computed. Polarization effects which determine Aiso as well as a simple model to describe the dipolar hfccs are discussed. All results are in excellent agreement with experimental data.
Molecular orbital study of proton hyperfine splitting constants in H 2CN radical
Theoretica Chimica Acta, 1963
By CARLO CORVAJA~ GIOVANNI GIACOMETTI arid PIERLUIGI ~ORDIO A hyperconjugated model is used to calculate the proton h. L s. constants in HeCN radical by the m. o. method. Agreement with experiment is excellent. A comparison between various approximations employed gives strong evidence of the importance of configuration interaction in such calculations. Mittels eines hyperkonjugierten Modells werden die Protonenhyperfeinstrukturkonstanten im H~CN-Radikal nach der MO-Methode bereehnet. Die Ubereinstimmung mit dem Experiment ist ausgezeichnet. Der Vergleich versehiedener Niiherungsmethoden beweist die Notwendigkeit, bei derartigen Reehnungen Konfigurationsweehselwirkung einzuschlieBen.
The Journal of Physical Chemistry, 1993
The structure and EPR parameters of dihydronitrosyl radical HzNO have been investigated by highly correlated ab-initio methods. The relative stabilities of planar and pyramidal structures have been analyzed in detail, taking also into account the effect of small-amplitude vibrations perpendicular to the inversion motion. Vibrational averaging of hyperfine coupling constants has been computed by a quantum-mechanical treatment based on the vibrational adiabatic zero curvature approximation. The general picture emerging from this study, substantiated by several checks, consists in a quasi-planar molecule with a nearly free inversion motion for out-of-plane angles as large as 30'. Due to compensation of different terms, vibrational averaging gives results very close to those obtained from a static treatment at an out-of-plane angle of about 20'. An equally important outcome of this work is the introduction of a general and reliable ab-initio strategy for the study of magnetic properties in nonrigid radicals.
Vibrationally averaged ?-hyperfine coupling constants for the muonium-substituted ethyl radical
Journal of The Chemical Society, Faraday Transactions, 1991
A variation-perturbation approach is applied to compute isotope-dependent zero-point vibrational corrections to the 8-hyperfine coupling constant of the ethyl radical. The corrections increase the coupling by ca. 5, 7 and 23% for the deuterium-, protium-and muonium-substituted radicals, respectively. The large vibrational correction to the muon 8-hyperfine coupling constant could explain the ' residual ' isotope effect observed experimentally.
ENDOR measurement of long-range hyperfine coupling in a nitroxyl radical
Journal of Magnetic Resonance (1969), 1980
ENDOR and EPR spectra of doxyl cyclododecane (3,3-dimethyl-l-oxa-l-azaspiro[4.1 llhexadec-4-yloxy) and the analog deuterated at the positions adjacent to the point of attachment of the doxyl ring yield hyperfine coupling values for seven of the protons in the molecule. Computer simulations using these results provide a more detailed understanding of the variable temperature EPR spectra.
Coupled-cluster studies of the hyperfine splitting constants of the thioformyl radical
The Journal of Chemical Physics, 2000
Hyperfine splitting constants ͑hfs͒ of the X 2 AЈ electronic ground state of the thioformyl radical ͑HCS͒ have been determined at the coupled-cluster level with single, double, and perturbatively applied connected triple excitations ͓CCSD͑T͔͒ using 39 basis sets. Variation of the CCSD͑T͒ hyperfine splittings with basis set was ascertained using a fixed geometry, optimized at the CCSD͑T͒ level with Dunning's correlation-consistent polarized valence quadruple-basis set ͑cc-pVQZ͒. Pople basis sets, 6-311Gϩϩ(2d,2p) and 6-311Gϩϩ(3d f ,3pd), give 1 H isotropic coupling constants (1 H A iso) in good agreement with the experimental vibrationally averaged value of 127.4 MHz, deviating by 5.5 and 9.3 MHz, respectively. Dunning's valence correlation-consistent basis sets ͑cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, aug-cc-pVTZ, cc-pVQZ, aug-cc-pVQZ͒ deviate 6.4 MHz ͑aug-cc-pVQZ͒ to 14.9 MHz ͑cc-pVDZ͒ from the experimental value. The correlation-consistent core valence analogues of these sets give very similar values with deviations from experiment of 7.4 MHz ͑cc-pCVQZ͒ to 14.2 MHz ͑cc-pCVDZ͒. A direct comparison with the vibrationally averaged experimental value is not precisely possible since the hyperfine splittings are strongly geometry dependent and all theoretical predictions refer to the equilibrium geometry. Small Pople basis sets ͑3-12G, 6-31G, and 6-311G͒ give the worst results, deviating by 49.5, 34.1, and 31.8 MHz, respectively. All CCSD͑T͒ 1 H A iso values fall below the experimental value. The 13 C and 33 S hyperfine splittings are not known experimentally, but the equilibrium values are predicted here to be 274.7 MHz (13 C) and 21.7 MHz (33 S) at the cc-pCVQZ CCSD͑T͒ level of theory. Significantly different values are predicted by density functional theory ͑DFT͒ for the 13 C and 33 S hyperfine splittings.
Study of the hyperfine coupling constants of the molecules NH2, NHD and ND2
Chemical Physics Letters, 1990
In the present paper we c:alculate tbe magnetic hyperfine couplina constants (hfcc) ai.ID and A 11 of the ground states of the isotopes NH 2 , NHD and ND 2 using truncated MR..CI methods. Differences from other theoretical methocls and shortoominp of the truncated Cl approach in calculating tlj 10 are studied. Polarization effects wbich detennirae ailo. as weU as a simple model to describe the dipolar hfcc's, are discussed. All results are in. excellent aareement with experimental data. lt is shown that ab initio methods are able to obtain reliable values for otf-diaaonal values of A 41 which are difficult to measure experimentaDy. l.lntroduction