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.     

The rotational spectrum of CuCCH(X̃ 1Σ+): a Fourier transform microwave discharge assisted laser ablation spectroscopy and millimeter/submillimeter study

The pure rotational spectrum of CuCCH in its ground electronic state (X? (1)Σ(+)) has been measured in the frequency range of 7-305 GHz using Fourier transform microwave (FTMW) and direct absorption millimeter/submillimeter methods. This work is the first spectroscopic study of CuCCH, a model system for copper acetylides. The molecule was synthesized using a new technique, discharge assisted laser ablation spectroscopy (DALAS). Four to five rotational transitions were measured for this species in six isotopologues ((63)CuCCH, (65)CuCCH, (63)Cu(13)CCH, (63)CuC(13)CH, (63)Cu(13)C(13)CH, and (63)CuCCD); hyperfine interactions arising from the copper nucleus were resolved, as well as smaller splittings in CuCCD due to deuterium quadrupole coupling. Five rotational transitions were also recorded in the millimeter region for (63)CuCCH and (65)CuCCH, using a Broida oven source. The combined FTMW and millimeter spectra were analyzed with an effective Hamiltonian, and rotational, electric quadrupole (Cu and D) and copper nuclear spin-rotation constants were determined. From the rotational constants, an r(m)(2) structure for CuCCH was established, with r(Cu-C) = 1.8177(6)?A?, r(C-C) = 1.2174(6)?A?, and r(C-H) = 1.046(2)?A?. The geometry suggests that CuCCH is primarily a covalent species with the copper atom singly bonded to the C≡C-H moiety. The copper quadrupole constant indicates that the bonding orbital of this atom may be sp hybridized. The DALAS technique promises to be fruitful in the study of other small, metal-containing molecules of chemical interest.     

The complex spectrum of a "simple" free radical: the Ã-X̃ band system of the jet-cooled boron difluoride free radical

The near-ultraviolet band system of the jet-cooled boron difluoride free radical has been studied by a combination of laser-induced fluorescence and single vibronic level wavelength resolved emission spectroscopies. The radical was produced in a supersonic discharge jet using a precursor mixture of 1%-3% of BF(3) or (10)BF(3) in high pressure argon. A large number of bands were found in the 340-286 nm region and assigned as transitions from the X?(2)A(1) ground state to the lower Renner-Teller component of the A?(2)Π excited state, based on our previous ab initio potential energy surface predictions, matching the emission spectra Franck-Condon profiles of (11)BF(2) and (10)BF(2), and comparison of observed and calculated boron isotope effects. Several bands have been rotationally analyzed providing ground state structural parameters of r(0)(') (BF) = 1.3102(9) ? and θ(0)(') (FBF) = 119.7(6)°. The ground state totally symmetric vibrational energy levels of both boron isotopologues have also been measured and assigned up to energies of more than 8000 cm(-1). Although BF(2) might be considered to be a "simple" free radical, understanding the details of its electronic spectrum remains a major challenge for both theory and experiment.     

The microwave rotational spectrum of X 2Σ LaO

The microwave absorption spectrum of LaO has been observed in the frequency range between 84 and 275 GHz. A precise set of Dunham coefficients for the X ²Σ⁺ ground state has been determined. Fine and hyperfine constants were found to be in agreement with previous data. An upper limit for the electric dipole moment of μ < 2 D can be estimated from the saturation of the transitions.     

Direct ab initio dynamics study of radical C4H (X̃2Σ+) + CH4 reaction

The methane (CH(4)) hydrogen abstraction reaction by linear butadiynyl radical C(4)H (CCCCH) has been investigated by direct ab initio dynamics over a wide temperature range of 100-3000 K, theoretically. The potential energy surfaces (PESs) have been constructed at the CCSD(T)/aug-cc-pVTZ//BB1K/6-311G(d,p) levels of theory. Two different hydrogen abstraction channels by C(1) and C(4) of C(4)H (C(1)C(2)C(3)C(4)H) have been considered. The results indicate that the C(1) position of C(4)H is a more reactive site. The electron transfer behaviors of two possible channels are also analyzed by quasi-restricted orbital (QRO) in detail. The rate constants calculated by canonical variational transition-state theory (CVT) with the small-curvature tunneling correction (SCT) are in excellent agreement with available experimental values. The normal and three-parameter expressions of Arrhenius rate constants are also provided within 100-3000 K. It is expected to be helpful for further studies on the reaction dynamics behaviors over a wide temperature range where no experimental data is available so far.     

Spectroscopic studies of the Ã-X̃ electronic spectrum of the β-hydroxyethylperoxy radical: structure and dynamics

The jet-cooled A?-X? near IR origin band spectra of the G(1)G(2)G(3) conformer of four β-hydroxyethylperoxy isotopologues, β-HEP (HOCH(2)CH(2)OO), β-DHEP (DOCH(2)CH(2)OO), β-HEP-d(4) (HOCD(2)CD(2)OO), and β-DHEP-d(4) (DOCD(2)CD(2)OO), have been recorded by a cavity ringdown spectrometer with a laser source linewidth of ~70 MHz. The spectra of all four isotopologues have been analyzed and successfully simulated with an evolutionary algorithm, confirming the cyclic structure of the molecule responsible for the observed origin band. The analysis also provides experimental A? and X? state rotational constants and the orientation of the transition dipole moment in the inertial axis system; these quantities are compared to results from electronic structure calculations. The observed, broad linewidth (Δν > 2 GHz) is attributed to a shortened lifetime of the A? state associated with dynamics along the reaction path for hydrogen transfer from the OH to OO group.     

The ν2 bending vibrational structure of the X̃2Σ+ state of MgNC

We have generated MgNC in supersonic free jet expansions and observed the laser induced fluorescence (LIF) of the A?(2)Π-X?(2)Σ(+) transition. We measured the LIF dispersed spectra from the single vibronic levels of the A?(2)Π electronic state of MgNC, following excitation of each ν(2) bending vibronic band observed, i.e., the κ series of the (0,v(2)('),0)-(0,0,0), v(2)(') = 0, 1, 2, 4, and 6 vibronic bands. In the vibrational structure in the dispersed fluorescence spectra measured, the long progression of the ν(2) bending mode in the X?(2)Σ(+) state is identified, e.g., up to v(2)(')=14 in the (0,6,0)-(0,v(2)('),0) spectrum. This enables us to derive the potential curve of the ν(2) bending mode in the X?(2)Σ(+) state. We used two kinds of models to obtain the potential curve; (I) the customary formula expressed in the polynomial series of the (v(2)(')+(d(2)/2)) term and (II) the internal rotation model. The potential curve derived from model (I) indicates the convergence of the bending vibrational levels at about 800 cm(-1) from the vibrationless level of MgNC, which may correspond to the barrier height of the isomerization reaction, MgNC ? MgCN, in the X?(2)Σ(+) state. Model (II) gives a simple picture for the isomerization reaction pathway with a barrier height of about 630 cm(-1) from the vibrationless level of the more stable species, MgNC. This shows that the v(2)(')=8 bending vibrational level of MgNC is already contaminated by the v(2)(')=2 bending vibrational level of the isomer, MgCN, and implies that the isomerization reaction begins at the v(2) (')=8 level. The bending potential surface and the isomerization reaction pathway, MgNC ? MgCN, in the X?(2)Σ(+) state are discussed by comparing the potential derived in this study with the surface obtained by quantum chemical calculation.     

Reaction of CN(X2Σ+) with OCS and formation of SCN

The reaction between CN(X²Σ⁺) and OCS has been investigated using both time-resolved spectrophotometry and flash spectroscopy. The reaction is shown to be fast (k ⩾ 3 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹) and to lead to the formation of SCN.     

The dipole moment of LiH(X1Σ+): Spin-coupled valence-bond study

The spin-coupled VB theory is applied to the dipole moment function μ(R) of LiH(X¹Σ⁺). A series of calculations is reported (1, 78, 127 and 188 spatial configurations) which demonstrates the rapid convergence of this property onto that given by accurate MO CI wavefunctions. The binding is essentially due to the exchange interaction between a heavily deformed Li valence orbital and an almost unchanged H(1s) function. The observed deformation of the Li(2s) orbital is due largely to acquisition of H(1s) character, and not to hydridisation.     

The pure rotational spectrum of HPS (X̃1A'): chemical bonding in second-row elements

The pure rotational spectrum of HPS, as well as its (34)S and D isotopologues, has been recorded at microwave, millimeter, and submillimeter wavelengths, the first observation of this molecule in the gas phase. The data were obtained using a combination of millimeter direct absorption, Fourier transform microwave (FTMW), and microwave-microwave double-resonance techniques, which cover the total frequency range from 15 to 419 GHz. Quantum chemical calculations at the B3LYP and CCSD(T) levels were also performed to aid in spectral identification. HPS was created in the direct absorption experiment from a mixture of elemental phosphorus, H(2)S, and Ar carrier gas; DPS was produced by adding D(2). In the FTMW study, these species were generated in a pulsed discharge nozzle from PH(3) and H(2)S or D(2)S, diluted in neon. The spectra recorded for HPS and its isotopologues exhibit clear asymmetric top patterns indicating bent structures; phosphorus hyperfine splittings were also observed in HPS, but not DPS. Analysis of the data yielded rotation, centrifugal distortion, and phosphorus nuclear spin-rotation parameters for the individual species. The r(m) ((1)) structure for HPS, calculated from the rotational constants, is r(H-P) = 1.438(1) A?, r(P-S) = 1.9320(1) A?, and θ(H-P-S) = 101.85(9)°. Empirically correcting for zero-point vibrational effects yields the geometry r(e)(H-P) = 1.4321(2) A?, r(e)(P-S) = 1.9287(1) A?, and θ(e)(H-P-S) = 101.78(1)°, in close agreement with the r(m) ((1)) structure. A small inertial defect was found for HPS indicating a relatively rigid molecule. Based on these data, the bonding in this species is best represented as H-P=S, similar to the first-row analog HNO, as well as HNS and HPO. Therefore, substitution of phosphorus and sulfur for nitrogen and oxygen does not result in a dramatic structural change.     

Alignment effect of N2(A3Σu+) in the energy transfer reaction of aligned N2(A3Σu+) + NO(X2Π) → NO(A2Σ+) + N2(X1Σg+)

Steric effect in the energy transfer reaction of N(2)(A(3)Σ(u)(+)) + NO(X(2)Π) → NO(A(2)Σ(+)) + N(2)(X(1)Σ(g)(+)) has been studied under crossed beam conditions at a collision energy of ~0.07 eV by using an aligned N(2)(A(3)Σ(u)(+)) beam prepared by a magnetic hexapole. The emission intensity of NO(A(2)Σ(+)) has been measured as a function of the magnetic orientation field direction (i.e., alignment of N(2)(A(3)Σ(u)(+))) in the collision frame. A significant alignment effect on the energy transfer probability is observed. The shape of the steric opacity function turns out to be most reactive at the oblique configuration of N(2)(A(3)Σ(u)(+)) with an orientation angle of γ(v(R)) ~ 45° with respect to the relative velocity vector (v(R)), which has a good correlation with the spatial distribution of the 2pπ(g)* molecular orbital of N(2)(A(3)Σ(u)(+)). We propose the electron exchange mechanism in which the energy transfer probability is dominantly controlled by the orbital overlap between N(2)(2pπ(g)*) and NO(6σ).     

The spectroscopic characterization of the methoxy radical. III. Rotationally resolved Ã2A1-X̃2E electronic and X̃2E submillimeter wave spectra of partially deuterated CH2DO and CHD2O radicals

Rotationally resolved laser induced fluorescence and stimulated emission pumping A?(2)A(1)-X?(2)E spectra, along with pure rotational spectra in the 153-263 GHz region within the E(3/2) component of the ground state in asymmetrically deuterated methoxy radicals CH(2)DO and CHD(2)O have been observed. The combined data set allows for the direct measurement with high precision of the energy separation between the E(1/2) and E(3/2) components of the ground state and the energy separation between the parity stacks in the E(3/2) component of the ground state. The experimentally observed frequencies in both isotopologues are fit to an effective rotational Hamiltonian accounting for rotational and spin-rotational effects arising in a near-prolate asymmetric top molecule with dynamic Jahn-Teller distortion. Isotopic dependencies for the molecular parameters have been successfully implemented to aid the analysis of these very complex spectra. The analysis of the first and second order contributions to the effective values of molecular parameters has been extended to elucidate the physical significance of resulting molecular parameters. Comparisons of measured parameters, e.g., spin-orbit coupling, rotational and spin-rotation constants, are made among the 5 methoxy isotopologues for which data is now available. Comparisons of experimental results, including the derived geometric structure at both the C(3v) conical intersection and at the Jahn-Teller distorted minima, are made with quantum chemistry calculations.     

The Ã-state dissociation continuum of NO-Ar and its near infrared spectrum

After preparing NO-Ar in a vibrational state correlating with the first overtone vibration in NO, we recorded its hot band UV spectrum by monitoring simultaneously the intensity in the NO(+) and the NO(+)-Ar ion channels. In this way, the bound as well as the continuous part of the electronic A?←X? spectrum are observed directly. Below the dissociation threshold, the intensity is found exclusively in the NO(+)-Ar ion channel while above it is found in the NO fragment ion channel. We observe simultaneously intensity in both ion channels only for a very narrow frequency range near the dissociation threshold. Structures in the dissociation spectrum correlate well with the thresholds for production of NO(A) in different rotational states. At frequencies well above the dissociation threshold, NO-Ar is detected efficiently as a NO fragment. This fact has been exploited to record the near IR spectrum of NO-Ar with significantly increased sensitivity. The dissociation detected spectra are essentially identical to our previous constant photon energy sum (CONPHOENERS) scans [B. Wen, Y. Kim, H. Meyer, J. K?os, and M. H. Alexander, J. Phys. Chem. A 112, 9483 (2008)]. Several hot band spectra have been remeasured with improved sensitivity enabling a comprehensive analysis yielding for the first time spectroscopic constants for levels associated with the potential surfaces of NO-Ar correlating with NO(v(NO) = 0 and 2). Since many NO-X complexes do not have a strong bound A?-state spectrum, although they do have a A?-state dissociation continuum, there is the possibility to record their near IR spectra by employing dissociation detection.     

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.     

The emission spectrum of AsH2(Ã2A1 → X̃2B1) via uv photolysis of AsH3

The AsH₂(òA₁ → X̃²B₁) emission spectrum (402–650 nm) following ArF laser photolysis of AsH₃ is reported. Seven bands are assigned and the relation ν = 19928 + (868v'₂-3v'₂²)-(987v″₂-6v″₂²) is obtained. The AsH₂ (òA₁) radiative lifetime is 130 ± 20 ns. Emission spectra from nascent As and a multiphoton ionization signal were also obtained.     

1 + 1] photodissociation of CS(2)(+)(X̃2Π(g)) via the vibrationally mediated B̃2Σ(u)+ state: multichannels exhibiting and mode specific dynamics

Dissociation dynamics of CS(2)(+) vibrationally mediated via its B?(2)Σ(u)(+) state, was studied using the time-sliced velocity map imaging technique. The parent CS(2)(+) cation was prepared in its X?(2)Π(g) ground state through a [3 + 1] resonance enhanced multiphoton ionization process, via the 4pσ(3)Π(u) intermediate Rydberg state of neutral CS(2) molecule at 483.14 nm. CS(2)(+)(X?(2)Π(g)) was dissociated by a [1?+?1] photoexcitation mediated via the vibrationally selected B? state over a wavelength range of 267-283 nm. At these wavelengths the C?(2)Σ(g)(+) and D?(2)Σ(u)(+) states are excited, followed by numerous S(+) and CS(+) dissociation channels. The S(+) channels specified as three distinct regions were shown with vibrationally resolved structures, in contrast to the less-resolved structures being presented in the CS(+) channels. The average translational energy releases were obtained, and the S(+)∕CS(+) branching ratios with mode specificity were measured. Two types of dissociation mechanisms are proposed. One mechanism is the direct coupling of the C? and D? states with the repulsive satellite states leading to the fast photofragmentation. The other mechanism is the internal conversion of the C? and D? states to the B? state, followed by the slow fragmentation occurred via the coupling with the repulsive satellite states.     

Detection of vibrational bending mode ν8 and overtone bands of the propargyl radical, HCCCH2 X̃ 2B1

Infrared (IR) absorption spectra of matrix-isolated HCCCH(2) have been measured. Propargyl radicals were generated in a supersonic pyrolysis nozzle, using a method similar to that described in a previous study (Jochnowitz, E. B.; Zhang, X.; Nimlos, M. R.; Varner, M. E.; Stanton, J. F.; Ellison, G. B. J. Phys. Chem. A 2005, 109, 3812-3821). Besides the nine vibrational modes observed in the previous study, this investigation detected the HCCCH(2) X? (2)B(1) out-of-plane bending mode (ν(8)) at 378.0 (±1.9) cm(-1) in a cryogenic argon matrix. This is the first experimental observation of ν(8) for the propargyl radical. In addition, seven overtone and combination bands have also been detected and assigned. Ab initio coupled-cluster anharmonic force field calculations were used to guide the analysis. Furthermore, ν(12), the HCCCH(2) in-plane bending mode, has been assigned to 333 (±10) cm(-1) based on the detection of its overtone (2ν(12), 667.7 ± 1.0 cm(-1)) and a possible combination band (ν(10) + ν(12), 1339.0 ± 0.8 cm(-1)). This is the first experimental estimation of ν(12) for the propargyl radical.     

Synthesis and microwave spectrum of vinyl isoselenocyanate (H2C═CHNCSe), a compound with a quasilinear CNCSe chain

The first α,β-unsaturated isoselenocyanate, vinyl isoselenocyanate (H(2)C═CHNCSe), has been synthesized, and its microwave spectrum has been investigated in the 11.5-77.0 GHz spectral range. The microwave work was augmented by quantum chemical calculations using four different methods, namely, CCSD(T), CCSD, B3LYP, and M062X, with the cc-pVTZ basis set. It is generally assumed that two rotamers having the isoselenocyanide group in an antiperiplanar or a synperiplanar position can exist for this compound. However, these four methods all predict that there is only one rotameric form of the molecule, namely, the antiperiplanar form. The CNC angle of the antiperiplanar rotamer is calculated to vary from 151° to 170° depending on the quantum chemical methodology. CCSD(T) and B3LYP potential functions of the in-plane CNC bending vibrations were calculated. These functions have one shallow minimum corresponding to the antiperiplanar form. The spectra of the ground and one vibrationally excited state of this rotamer were assigned. Spectral searches for the synperiplanar form were performed but were not successful, so this form must have a relatively high energy, if it exists at all. The vibrationally excited state is presumably the lowest in-plane bending vibration of the CNC angle. Relative intensity measurements yielded a very low frequency of 18(25) cm(-1) for this vibration. The large-amplitude vibration of this mode suggests that this compound should rather be regarded as having a quasilinear CNCSe link of atoms than a rigid, bent antiperiplanar form.     

The pure rotational spectrum of the CrS radical in its X 5Π(r) state

The pure rotational spectrum of the CrS radical has been measured in its ground X (5)Π(r) state using gas-phase millimeter/submillimeter direct absorption methods. The molecule was created by the reaction of chromium vapor, sublimed in a Broida-type oven, with hydrogen sulfide. Eleven rotational transitions were recorded for this free radical in the frequency range of 280-405 GHz; in most transitions, all five spin components were observed, and lambda-doubling was resolved in the Ω=0, 1, and 2 ladders. The data were fit with a Hund's case (a) Hamiltonian and rotational, spin-orbit, spin-spin, and lambda-doubling constants were established. Higher order spin and spin-orbit terms were essential in the analysis. The lambda-doubling constants indicate a nearby (5)Σ(+) state at an energy of ～1500-2000?cm(-1). A bond length of 2.0781 A? was derived for CrS from the data, which is larger than the value of 2.0682 A? found for MnS by ～0.01?A?. In contrast, the bond distance for MnO is greater than that of CrO by 0.03 A?, an illustration of the subtle differences between 3d oxide and sulfides. CrS is the second molecule in a (5)Π state that has been studied by rotational spectroscopy.