The microwave and millimeter spectrum of ZnCCH (X̃2Σ+): a new zinc-containing free radical

The pure rotational spectrum of the ZnCCH (X?(2)Σ(+)) radical has been measured using Fourier transform microwave (FTMW) and millimeter direct-absorption methods in the frequency range of 7-260 GHz. This work is the first study of ZnCCH by any type of spectroscopic technique. In the FTMW system, the radical was synthesized in a mixture of zinc vapor and 0.05% acetylene in argon, using a discharge assisted laser ablation source. In the millimeter-wave spectrometer, the molecule was created from the reaction of zinc vapor, produced in a Broida-type oven, with pure acetylene in a dc discharge. Thirteen rotational transitions were recorded for the main species, (64)ZnCCH, and between 4 and 10 for the (66)ZnCCH, (68)ZnCCH, (64)ZnCCD, and (64)Zn(13)C(13)CH isotopologues. The fine structure doublets were observed in all the data, and in the FTMW spectra, hydrogen, deuterium, and carbon-13 hyperfine splittings were resolved. The data have been analyzed with a (2)Σ Hamiltonian, and rotational, spin-rotation, and H, D, and (13)C hyperfine parameters have been established for this radical. From the rotational constants, an r(m) ((1)) structure was determined with r(Zn-C) = 1.9083 A?, r(C-C) = 1.2313 A?, and r(C-H) = 1.0508 A?. The geometry suggests that ZnCCH is primarily a covalent species with the zinc atom singly bonded to the C≡C-H moiety. This result is consistent with the hyperfine parameters, which suggest that the unpaired electron is localized on the zinc nucleus. The spin-rotation constant indicates that an excited (2)Π state may exist ～19,000 cm(-1) in energy above the ground state.     

The microwave spectrum of the 1,1-difluoroprop-2-ynyl radical, F2*C-C[triple bond]CH

The rotational spectrum of the 1,1-difluoroprop-2-ynyl radical, F2*C-C[triple bond]CH, a partially fluorinated variant of the propargyl radical, has been recorded in the ground electronic, 2B1, state using pulsed discharge, pulsed-jet, Fabry-Perot Fourier transform microwave spectroscopy. Five successive a-type rotational transitions, from N = 1-0 to N = 5-4, and Ka = 0, 1, and 2, were measured between 6.5 and 32.5 GHz with an uncertainty of 5 kHz. The molecular constants, including fine and hyperfine constants, were precisely determined. These constants are compared with our predictions based on a density functional theory level ab initio calculations and with the fine and hyperfine constants of the propargyl radical. The measured electron spin densities suggest that both the difluoropropargyl and the difluoroallenyl resonance forms [F2*C-C[triple bond]CH<-->F2C=C=C*H] make major contributions to the electronic structure of the radical.     

Structure determination of beta-Pb2ZnF6 by coupling multinuclear solid state NMR, powder XRD and ab initio calculations

The results from one-dimensional multinuclear (19F, 207Pb and 67Zn) magic-angle spinning nuclear magnetic resonance experiments combined with the use of the ISODISPLACE program allow for the space group determination of beta-Pb2ZnF6 (no. 138 P4(2)/ncm). The structure was refined from X-ray powder diffraction data (a = 5.633 (1) A and c = 16.247 (1) A, Z = 4). beta-Pb2ZnF6 has one six-fold coordinated Zn, one eleven-fold coordinated Pb and five F non-equivalent crystallographic sites and is built from alternated layers parallel to the (a, b) plane; tilted ZnF4(2-) layers of corner sharing ZnF6(4-) octahedra and FPb+ layers of edge sharing FPb4(7+) tetrahedra. The structure of beta-Pb2ZnF6 was then optimized using the ab initio code WIEN2k and the calculated 67Zn EFG is in agreement with the NMR results. 19F-19F proximities and 19F-207Pb connectivities were evidenced using through-space and through-bond NMR correlation experiments, respectively, and support the proposed structure. 19F-207Pb J-coupling was also used to select fluorine resonances depending on the number of neighbouring lead ions, leading to an unambiguous assignment of the different 19F resonances.     

The vibration-rotation emission spectrum of gaseous HZnCl

Gaseous HZnCl has been synthesized for the first time in a high-temperature tube furnace with a dc discharge in a flowing mixture of pure HCl and Zn vapor. The vibration-rotation emission spectrum of HZnCl was recorded at high resolution using a Fourier transform spectrometer. The H-Zn stretching modes (nu(1)) of the H(64)Zn(35)Cl, H(66)Zn(35)Cl, H(68)Zn(35)Cl, and H(64)Zn(37)Cl species, as well as the 2nu(1)-nu(1) hot band of the most abundant isotopologue H(64)Zn(35)Cl, were observed near 1966 cm(-1). A least-squares fit was performed for each of the four observed isotopologues, and their spectroscopic constants were determined.     

Determination of the proton tunneling splitting of the vinyl radical in the ground state by millimeter-wave spectroscopy combined with supersonic jet expansion and ultraviolet photolysis

The vinyl radical in the ground vibronic state produced in a supersonic jet expansion by 193 nm excimer laser photolysis of vinyl bromide was investigated by millimeter-wave spectroscopy. Due to the proton tunneling, the ground state is split into two components, of which the lower and higher ones are denoted as 0+ and 0-, respectively. Eight pure rotational transitions with Ka = 0 and 1 obeying a-type selection rules were observed for each of the 0+ and 0- states in the frequency region of 60-250 GHz. Tunneling-rotation transitions connecting the lower (0+) and upper (0-) components of the tunneling doublet, obeying b-type selection rules, were also observed in the frequency region of 190-310 GHz, including three R- and six Q-branch transitions. The observed frequencies of the pure rotational and tunneling-rotation transitions were analyzed by using an effective Hamiltonian in which the coupling between the 0+ and 0- states was taken into account. A set of precise molecular constants was obtained. Among others, the proton tunneling splitting in the ground state was determined to be DeltaE0 = 16,272(2) MHz. The potential barrier height was estimated to be 1580 cm(-1) from the proton tunneling splitting, by an analysis using a detailed one-dimensional model. The spin-rotation and hyperfine interaction constants were also determined for the 0+ and 0- states together with the off-diagonal interaction constants connecting the 0+ and 0- states, epsilonab + epsilonba for the spin-rotation interaction and Tab for the hyperfine interaction of the alpha (CH) proton. The hyperfine interaction constants, due to the alpha proton and the beta (CH2) protons, are consistent with those derived from electron spin resonance studies.     

Spectroscopy and electronic structure of electron deficient zinc phthalocyanines

The effect of introduction of perfluoro alkyl groups into phthalocyanines, as evidenced by the spectroscopic properties of 1,4,8,11,15,18,22,25-octa-fluoro-2,3,9,10,16,17,23,24-octa-perfluoro isopropyl zinc phthalocyanine, ZnF(64)Pc(-2) and its ring-reduced radical anion species, [ZnF(64)Pc(-3)](-), are reported. A combination of UV-visible absorption and magnetic circular dichroism (MCD) spectroscopy, ESI and MALDI-TOF mass spectrometry, cyclic and differential pulse voltammetry, and complete theoretical calculations using INDO/S and DFT techniques reveals that the substitution of all sixteen hydrogen atoms in protio ZnPc(-2) by eight F and eight i-C(3)F(7) groups red shifts the Q and pi --> pi transitions and narrows the HOMO-LUMO gap while simultaneously preventing ring photooxidation and stabilizing the radical anion. The [ZnF(64)Pc(-3)](-) species, which is in equilibrium in solution with the neutral complex when a reducing agent is present, is unusually stable. The above effects are attributed to the strong electron withdrawing properties of the peripheral substituents, which render ZnF(64)Pc extremely electron deficient.     

Hyperfine resolved spectrum of the bromomethyl radical, CH2Br, by Fourier transform microwave spectroscopy

Pure rotational spectra of the bromomethyl radical, CH(2)Br, were measured by using a Fourier transform microwave (FT-MW) spectrometer in order to fully resolve hyperfine structures arising from both the bromine and hydrogen nuclei. We detected a total of 124 lines for the (79)Br and (81)Br isotopomers, including K(a)=0 (ortho species) and K(a)=1 (para species). No hyperfine splitting due to the hydrogen nuclei was observed for the para species, directly confirming the planarity of the radical. We conducted a global analysis of our present FT-MW results and previous measurements in the millimeter-wave region and obtained an exhaustive list of molecular constants. The sign of the Fermi constant of the bromine nucleus was unambiguously determined to be positive, which is opposite to that found in previous work in the millimeter-wave region and in electron spin resonance experiment on this radical. The present study permitted a systematic comparison to be made of the hyperfine coupling constants of both the halogen and hydrogen nuclei for CH(2)X-type compounds, where X=F, Cl, and Br.     

Microwave survey of the conformational landscape exhibited by the propeller molecule triethyl amine

Conformational studies with quantum chemical methods yielded for the most stable conformer of triethyl amine a propeller-like structure belonging to the point group C(3), which corresponds to an oblate top. The microwave spectrum of this conformer with (14)N hyperfine splitting of all rotational transitions was assigned and molecular parameters were determined. The rotational constants were found to be A = B = 2.314873978(11) GHz, the (14)N quadrupole coupling constant χ(cc) = -5.2444(07) MHz. The observed spectrum could be reproduced within experimental accuracy. The standard deviation of a global fit with 48 rotational transitions is 1.5 kHz. The propeller-like structure seems to be energetically favorable and therefore also typical for related systems like triethyl phosphine, triisopropyl amine, tri-n-propyl amine, and tri-tert-butyl amine. Furthermore, the rotational transitions of two isotopologues, (13)C(2) and (13)C(5), could be measured in natural abundance and fitted with an excellent standard deviation. The C rotational constants could be determined to be 1.32681(96) GHz and 1.32989(18) GHz for the (13)C(2) and (13)C(5) isotopologues, respectively.     

Hyperfine structure observed in EPR spectra of polyphenylacetylene solutions

The high-temperature (>120C) electron paramagnetic resonance (EPR) spectrum of solutions of polyphenylacetylene have been deconvoluted into the spectra of two separate radicals, a delocalized π radical, whose EPR spectrum consists of a single 15-G wide Gaussian line comprising about 90% of the total signal and a second, more localized π radical exhibiting complex hyperfine structure in its EPR spectrum. Some possible structures for the minor component radical are suggested and their hyperfine splitting constants calculated using molecular orbital theory.     

The microwave and millimeter rotational spectra of the PCN radical (X3Σ-)

The pure rotational spectrum of the PCN radical (X(3)Σ(-)) has been measured for the first time using a combination of millimeter/submillimeter direct absorption and Fourier transform microwave (FTMW) spectroscopy. In the millimeter instrument, PCN was created by the reaction of phosphorus vapor and cyanogen in the presence of an ac discharge. A pulsed dc discharge of a dilute mixture of PCl(3) vapor and cyanogen in argon was the synthetic method employed in the FTMW machine. Twenty-seven rotational transitions of PCN and six of P(13)CN in the ground vibrational state were recorded from 19 to 415 GHz, all which exhibited fine structure arising from the two unpaired electrons in this radical. Phosphorus and nitrogen hyperfine splittings were also resolved in the FTMW data. Rotational satellite lines from excited vibrational states with v(2) = 1-3 and v(1) = 1 were additionally measured in the submillimeter range. The data were analyzed with a Hund's case (b) effective Hamiltonian and rotational, fine structure, and hyperfine constants were determined. From the rotational parameters of both carbon isotopologues, the geometry of PCN was established to be linear, with a P-C single bond and a C-N triple bond, structurally comparable to other non-metal main group heteroatom cyanides. Analysis of the hyperfine constants suggests that the two unpaired electrons reside almost exclusively on the phosphorus atom in a π(2) configuration, with little interaction with the nitrogen nucleus. The fine structure splittings in the vibrational satellite lines differ significantly from the pattern of the ground state, with the effect most noticeable with increasing v(2) quantum number. These deviations likely result from spin-orbit vibronic perturbations from a nearby (1)Σ(+) state, suggested by the data to lie ~12,000 cm(-1) above the ground state.     

Characterisation of H2S···CuCl and H2S···AgCl isolated in the gas phase: a rigidly pyramidal geometry at sulphur revealed by rotational spectroscopy and ab initio calculations

Pure rotational spectra of the ground vibrational states of eight isotopologues of H(2)S···CuCl and twelve isotopologues of H(2)S···AgCl have been analysed allowing rotational constants and hyperfine coupling constants to be determined. The molecular structures have been determined from the measured rotational constants and are presented alongside the results of calculations at the CCSD(T) level. Both molecules have C(s) symmetry at equilibrium and are pyramidal at the sulphur atom. The chlorine, metal, and sulphur atoms are collinear while the local C(2) axis of the hydrogen sulphide molecule intersects the axis defined by the heavy atoms at an angle, φ = 74.46(2)° for Cu and φ = 78.052(6)° for Ag. The molecular geometries are rationalised using simple rules that invoke the electrostatic interactions within the complexes. Centrifugal distortion constants, Δ(J), and nuclear quadrupole coupling constants, χ(aa)(Cu) and χ(aa)(Cl) for H(2)S···CuCl are presented for the first time. The geometry of H(2)S···AgCl is determined with fewer assumptions and greater precision than previously.     

Electron paramagnetic resonance investigation of the complexes of grignard reagents with 9,10-phenanthrenesemiquinonate

The substituent effect on the proton hyperfine splitting constants (hfsc) of the charge-transfer complexes of Grignard reagents (RMgBr, R = CH₃, C₂H₅, CH₃CHCH) with seven 3-substituted 9,10-phenanthroquinones is reported. A back-donation bonding interaction between phenanthrenesemiquinonate anion radical π^* and the magnesium p orbital is revealed by the hfsc analysis.     

Influence of conformation on the EPR spectrum of 5,5-dimethyl-1-hydroperoxy-1-pyrrolidinyloxyl: a spin trapped adduct of superoxide

Spin trapping, a technique used to characterize short-lived free radicals, consists of using a nitrone or nitroso compound to "trap" an unstable free radical as a long-lived aminoxyl that can be characterized by EPR spectroscopy. The resultant aminoxyl exhibits hyperfine splitting constants that are dependent on the spin trap and the free radical. Such is the case with 2,2-dimethyl-5-hydroxy-1-pyrrolidinyloxyl (DMPO-OH) and 2,2-dimethyl-5-hydroperoxy-1-pyrrodinyloxyl (DMPO-OOH) whose hyperfine splitting constants, A(N) = A(H) = 14.9 G and A(N) = 14.3 G, A(H)(beta) = 11.7 G, and A(H)(gamma) = 1.25 G, respectively, have been used to demonstrate the generation of HO(*) and O(2)(*)(-). However, to date, the source of the apparent A(H)(gamma) hyperfine splitting in DMPO-OOH is not known. We consider three possible explanations to account for the unique EPR spectrum of DMPO-OOH. The first is that the gamma-splitting arises from one of the hydrogen atoms at either carbon 3 or carbon 4 of DMPO-OOH. The second is that the gamma-splitting originates from the hydrogen atom of DMPO-OOH. The third is that the conformational properties of DMPO-R change upon going from DMPO-OH to DMPO-OOH. Experimental and theoretical chemical approaches as well as EPR spectral modeling were used to investigate which of these hypotheses may explain the asymmetric EPR spectrum of DMPO-OOH. From these studies it is shown that the 12-line EPR spectrum of DMPO-OOH results not from any proximal hydrogen, but from additional conformers of DMPO-OOH. Thus, the 1.25 G hyperfine splitting, which has been assigned as a gamma-splitting, is actually from two individual EPR spectra associated with different conformers of DMPO-OOH.     

High-resolution electron spin resonance spectroscopy of XeF* in solid argon. The hyperfine structure constants as a probe of relativistic effects in the chemical bonding properties of a heavy noble gas atom

Xenon fluoride radicals were generated by solid-state chemical reactions of mobile fluorine atoms with xenon atoms trapped in Ar matrix. Highly resolved electron spin resonance spectra of XeF* were obtained in the temperature range of 5-25 K and the anisotropic hyperfine parameters were determined for magnetic nuclei 19F, 129Xe, and 131Xe using naturally occurring and isotopically enriched xenon. Signs of parallel and perpendicular hyperfine components were established from analysis of temperature changes in the spectra and from numerical solutions of the spin Hamiltonian for two nonequivalent magnetic nuclei. Thus, the complete set of components of hyperfine- and g-factor tensors of XeF* were obtained: 19F (Aiso=435, Adip=1249 MHz) and 129Xe (Aiso=-1340, Adip=-485 MHz); g(parallel)=1.9822 and g(perpendicular)=2.0570. Comparison of the measured hyperfine parameters with those predicted by density-functional theory (DFT) calculations indicates, that relativistic DFT gives true electron spin distribution in the 2Sigma+ ground-state, whereas nonrelativistic theory underestimates dramatically the electron-nuclear contact Fermi interaction (Aiso) on the Xe atom. Analysis of the obtained magnetic-dipole interaction constants (Adip) shows that fluorine 2p and xenon 5p atomic orbitals make a major contribution to the spin density distribution in XeF*. Both relativistic and nonrelativistic calculations give close magnetic-dipole interaction constants, which are in agreement with the measured values. The other relativistic feature is considerable anisotropy of g-tensor, which results from spin-orbit interaction. The orbital contribution appears due to mixing of the ionic 2Pi states with the 2Sigma+ ground state, and the spin-orbit interaction plays a significant role in the chemical bonding of XeF*.     

Theoretical studies of alkyl radicals in the NaY and HY zeolites

Interplay of quantum mechanical calculations and experimental data on hyperfine coupling constants of ethyl radical in zeolites at several temperatures was engaged to study the geometries and binding energies and to predict the temperature dependence of hyperfine splitting of a series of alkyl radicals in zeolites for the first time. The main focus is on the hyperfine interaction of alkyl radicals in the NaY and HY zeolites. The hyperfine splitting for neutral free radicals and free radical cations is predicted for different zeolite environments. This information can be used to establish the nature of the muoniated alkyl radicals in the NaY and HY zeolites via muSR experiments. The muon hyperfine coupling constants of the ethane radical cation in these zeolites are very large with relatively little dependence on temperature. It was found that the intramolecular dynamics of alkyl free radicals are only weakly affected by their strong binding to zeolites. In contrast, the substrate binding has a significant effect on their intermolecular dynamics.     

The fluoroformyloxyl radical geometry and ground-state rotational spectra of the free FC18O2· radical

The rotational spectra of the isotopically substituted free fluoroformyloxyl radical FC(18)O(2·) were measured using the Prague millimeter-wave high-resolution spectrometer. More than 110 rotational-fine-hyperfine transition lines were observed and assigned to appropriate quantum numbers in the frequency range of 235-270 GHz. The obtained transition frequencies were analyzed with standard effective Hamiltonians to acquire a set of precise rotational, centrifugal-distortion, fine, and hyperfine structure molecular constants. Merging the new FC(18)O(2·) isotopologue molecular parameters with those previously obtained for the ordinary FC(16)O(2)[middle dot] radical, the substitution molecular geometry in the ground vibronic state was evaluated. The molecular parameters for both radical isotopologues were also calculated by several quantum chemistry methods and their calculated mutual ratios are in remarkable agreement with the experimental FC(16)O(2·)/FC(18)O(2·) parameter ratios. The measurements, assignments of the 18-oxygen isotopologue FC(18)O(2·) radical millimeter-wave transitions, as well as the derivation of the fluoroformyloxyl radical ground-state geometry have been carried out for the first time.     

Microwave rotational spectra and structures of 2-fluoropyridine and 3-fluoropyridine

The ground state rotational spectra of 2-fluoropyridine and 3-fluoropyridine have been investigated using both Fourier transform microwave (FTMW) and chirped pulse Fourier transform microwave (cp-FTMW) spectroscopies. In addition to the parent species, the spectra of the (13)C and (15)N singly substituted isotopologues were recorded in the 8-23 GHz region in natural abundance. The rotational constants determined for the seven isotopologues of each were used to calculate relevant geometric parameters including the bond distances and angles of the pyridine ring backbone. The derived structures show a more pronounced deviation from the pyridine ring geometry when the fluorine substituent is ortho to nitrogen which is consistent with ab initio predictions at various levels of theory. Analysis of the (14)N hyperfine structure provided an additional source of information about the electronic structure surrounding the nitrogen atom as a function of fluorine substitution. Together, the experimental results are consistent with a bonding model that involves hyperconjugation whereby fluorine donates electron density from its lone pair into the π-system of pyridine.     

Solution combustion derived nanocrystalline Zn(2)SiO(4):Mn phosphors: a spectroscopic view

Manganese doped nanocrystalline willemite powder phosphors Zn(2-x)Mn(x)SiO(4) (0.1(6)A(1) ground state. The mechanism involved in the generation of a green emission has been explained in detail. The effect of Mn content on luminescence has also been studied.     

Millimeter-wave spectroscopy of H(2)C=CD: Tunneling splitting and ortho-para mixing interaction

The H(2)C=CD isotopic species of vinyl radical produced in a supersonic jet expansion by ultraviolet laser photolysis was studied by millimeter-wave spectroscopy. Due to the tunneling motion of the α deuteron, the ground state is split into two components, 0(+) and 0(-). Tunneling-rotation transitions connecting the lower (0(+)) and upper (0(-)) components of the tunneling doublet were observed in the frequency region of 184-334 GHz, including three R- and two Q-branch transitions. Three and two pure rotational transitions in the K(a)=0 and 1 stacks, respectively, were also observed for each of the 0(+) and 0(-) states in the frequency region of 52-159 GHz. Least-squares analysis of the observed frequencies for the tunneling-rotation and pure rotational transitions with well resolved hyperfine structures yielded a set of precise molecular constants, among which the tunneling splitting in the ground state was determined to be ΔE(0)=1187.234(17)?MHz, which is 1/14 that for H(2)C=CH. The potential barrier height derived from the observed tunneling splitting by an analysis of the tunneling dynamics using a one-dimensional model is 1545?cm(-1), consistent with the value 1568?cm(-1) obtained for the normal vinyl. The observed spectrum was found to be perturbed by a hyperfine interaction connecting ortho and para levels. The constant for the interaction, which we call the ortho-para mixing Fermi contact interaction, has been determined to be δa(F) ((β))=68.06(53)?MHz. This is believed to be the first definite detection of such an interaction. By this interaction the ortho and para states of H(2)C=CD are mixed up to about 0.1%. The constant is more than 1000 times larger than spin-rotation interaction constants that cause ortho-para mixing in closed shell molecules and suggests extremely rapid conversion between the ortho and para nuclear spin isomers of H(2)C=CD.