Electronic structure of simple phosphorus containing molecules [C,xH,O,P] candidate for astrobiology (x=1, 3, 5)

The present report is a prospective study aimed at finding phosphorus containing compounds for astrobiology. Since PN, PC and HCP are the only species detected so far, it was deemed reasonable to enlarge the quest for phosphorus compounds to mixed carbon oxygen containing compounds [C,xH,O,P] analogue to the CHON family. Ab initio M?ller-Plesset (MP2), Coupled Cluster (CCSD(T)) and Density Functional Theory (DFT) were used. State of the art level of theory, CCSD(T)/cc-pVQZ, was necessary to show that CH3-PH2=O is the most stable isomer, with CH3-PH-OH close by in the [C,5H,O,P] sub-family. This structure has the same C-P-O connectivity as the most stable compound of the [C,3H,O,P] sub-family, CH3-P=O but differs from the simplest [C,H,O,P] system HP=C=O. Rotational constants B=7.1377 and C=6.0636 GHz associated with a dipole moment of 4.2 Debye together with an IR spectrum with very strong bands at 1214, 2282, 2264 and 1039 cm(-1) have been calculated for CH3-PH2=O. For CH3-P=O, one has B=7.9881 and C=6.4659 GHz, a dipole moment of 2.9 Debye and four IR bands at 1198, 623, 835, 1256 cm(-1) of medium intensity. The simplest HPCO system with B=5.5206 and 5.3952 GHz and a dipole moment of 0.8 Debye has only one very strong IR frequency at 2037 cm(-1). The above values should be precise enough to encourage laboratory experiments on these prototype molecules.     

A priori predictions of the rotational constants for protonated formaldehyde and protonated methanol

Protonated formaldehyde and protonated methanol are candidate interstellar molecules and models for classes of protonated oxygen compounds. Ab initio molecular orbital theory has been used to compute rotational constants to guide spectroscopic searches both in the laboratory and in space. The ab initio results are empirically corrected to account for systematic deficiencies in the theory and zero-point vibrational effects; they are expected to be accurate to approximately +/-2%. For H2COH+ the resultant constants are (in GHz) A = 194.3, B = 34.28, and C = 29.14; for H3COH2+ A = 103.7, B = 21.18, and C = 20.30.     

Gas-phase detection of HSOH: synthesis by flash vacuum pyrolysis of di-tert-butyl sulfoxide and rotational-torsional spectrum

Gas-phase oxadisulfane (HSOH), the missing link between the well-known molecules hydrogen peroxide (HOOH) and disulfane (HSSH), was synthesized by flash vacuum pyrolysis of di-tert-butyl sulfoxide. Using mass spectrometry, the pyrolysis conditions have been optimized towards formation of HSOH. Microwave spectroscopic investigation of the pyrolysis products allowed-assisted by high-level quantum-chemical calculations--the first measurement of the rotational-torsional spectrum of HSOH. In total, we have measured approximately 600 lines of the rotational-torsional spectrum in the frequency range from 64 GHz to 1.9 THz and assigned some 470 of these to the rotational-torsional spectrum of HSOH in its ground torsional state. Some 120 out of the 600 lines arise from the isotopomer H(34)SOH. The HSOH molecule displays strong c-type and somewhat weaker b-type transitions, indicating a nonplanar skew chain structure, similar to the analogous molecules HOOH and HSSH. The rotational constants (MHz) of the main isotopomer (A=202 069, B=15 282, C=14 840), determined by applying a least-squares analysis to the presently available data set, are in excellent agreement with those predicted by quantum-chemical calculations (A=202 136, B=15 279, C=14 840). Our theoretical treatment also derived the following barrier heights against internal rotation in HSOH (when in the cis and trans configurations) to be V(cis) approximately equal to 2216 cm(-1) and V(trans) approximately equal to 1579 cm(-1). The internal rotational motion results in detectable torsional splittings that are dependent on the angular momentum quantum numbers J and K(a).     

Arsenic in prebiotic species: a theoretical approach

A recent controversy about the presence of arsenic in biological systems prompted us to investigate the possible replacement of phosphorus by arsenic in prebiotic species small enough to be potentially identified in space. Systematic computational experiments were carried out on simple systems able to form a peptide or analogous bond. Density Functional Theory (DFT) within the B3LYP formalism, MP2 and CCSD(T) methods were used to determine the most stable isomers that can possibly form from the [C,H,O,As] and [C,3H,O,As] sets of atoms. It was found that HAsCO, like HPCO and HNCO was the most stable isomer. With three hydrogen atoms, the peptide-like bond (AsH(2)-CH=O) is not the most stable structure, contrary to NH(2)-CH=O. It is ～9 kcal mol(-1) higher than the most stable structure, CH(2)[double bond, length as m-dash]As-OH. To assess the plausibility of the As to P substitution, a comparative study of the dimethylphosphate (DMP) and dimethylarsenate (DMA) anions was then carried out. It was found that the gauche-gauche arrangement that mimics the helix structure is the most stable one in both model molecules, showing that there is no structural evidence to discard the hypothesis of the possible inclusion of As in place of P in the DNA architecture. The topological analysis of the ELF function showed a weakening by 50% of two As-O covalent bonds in all the DMA conformers. It means that if As replaces P, the structure of the DNA helix could be weakened. Rotational constants and IR frequencies of the low-lying isomers are given to encourage laboratory experiments on these prototype molecules.     

Femtosecond degenerate four-wave mixing of carbon disulfide: high-accuracy rotational constants

Femtosecond degenerate four-wave mixing (fs-DFWM) rotational coherence spectroscopy (RCS) has been used to determine the rotational and centrifugal distortion constants of the 00 (0)0 ground and 01 (1)0 vibrationally excited states of gas-phase CS(2). RCS transients were recorded over the 0-3300 ps optical delay range, allowing the observation of 87 recurrences. The fits yield rotational constants B(00 (0)0)=3.271 549 2(18) GHz for (12)C(32)S(2) and B(00 (0)0)=3.175 06(21) GHz for the (12)C(32)S(34)S isotopomer. The rotational constants of the degenerate 01 (1)0 bending level of (12)C(32)S(2) are B(01 (1)0)=3.276 72(40) and 3.279 03(40) GHz for the e and f substrates, respectively. These fs-DFWM rotational constants are ten times more accurate than those obtained by CO(2) laser/microwave heterodyne measurements and are comparable to those obtained by high-resolution Fourier transform infrared spectroscopy. Ab initio calculations were performed at two levels, second-order Moller-Plesset theory and coupled-cluster singles, doubles, and iterative triples [CCSD(T)]. The equilibrium and vibrationally averaged C=S distances were calculated using large Dunning basis sets. An extrapolation procedure combining the ab initio rotational constants with the experiment yields an equilibrium C=S bond length of 155.448 pm to an accuracy of +/-20 fm. The theoretical C=S bond length obtained by a complete basis set extrapolation at the CCSD(T) level is r(e)(C=S)=155.579 pm, or 0.13 pm longer than that in the experiment.     

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.     

Fourier transform microwave and millimeter wave spectroscopy of quinazoline, quinoxaline, and phthalazine

The pure rotational spectra of the bicyclic aromatic nitrogen heterocycle molecules, quinazoline, quinoxaline, and phthalazine, have been recorded and assigned in the region 13-87 GHz. An analysis, guided by ab initio molecular orbital predictions, of frequency-scanned Stark modulated, jet-cooled millimeter wave absorption spectra (48-87 GHz) yielded a preliminary set of rotational and centrifugal distortion constants. Subsequent spectral analysis at higher resolution was carried out with Fourier transform microwave (FT-MW) spectroscopy (13-18 GHz) of a supersonic rotationally cold molecular beam. The high spectral resolution of the FT-MW instrument provided an improved set of rotational and centrifugal distortion constants together with nitrogen quadrupole coupling constants for all three species. Density functional theory calculations at the B3LYP∕6-311+G?? level of theory closely predict rotational constants and are useful in predicting quadrupole coupling constants and dipole moments for such species.     

Millimetre wave spectroscopy of PANHs: phenanthridine

The pure rotational spectrum of phenanthridine (C(13)H(9)N), a small polycyclic aromatic nitrogen heterocycle (PANH), has been measured from 48 to 85 GHz employing Stark modulated millimetre wave absorption spectroscopy of a supersonic rotationally cold molecular beam. Initial survey search scans were guided by rotational constants obtained through quantum chemical calculations performed at the B3LYP/cc-pVTZ level of theory. Close agreement--to well within 1%--is found between the calculated equilibrium and experimentally derived ground state rotational constants. From the moments of inertia a substantial negative inertial defect of Delta = -0.4688(44) amu Angstroms(2) is obtained which can be explained by the presence of several energetically low-lying out-of-plane vibrational modes. Corresponding density functional theory calculations of harmonic fundamental frequencies indeed yield four such low frequency modes with values as low as 96 cm(-1). The data presented here will also be useful for deep radio astronomical searches for PANHs employing large radio telescopes.     

Analysis of the rotational structure in the high-resolution infrared spectrum of trans-hexatriene-1-13C1: a semiexperimental equilibrium structure for the C6 backbone of trans-hexatriene

trans-Hexatriene-1-(13)C(1) (tHTE-1-(13)C(1)) has been synthesized, and its high-resolution (0.0015 cm(-1)) infrared spectrum has been recorded. The rotational structure in the C-type bands for ν(26) at 1011 cm(-1) and ν(30) at 894 cm(-1) has been analyzed. To the 1458 ground state combination differences from these bands, ground state rotational constants were fitted to a Watson-type Hamiltonian to give A(0) = 0.8728202(9), B(0) = 0.0435868(4), and C(0) = 0.0415314(2) cm(-1). Upper state rotational constants for the ν(30) band were also fitted. Predictions of the ground state rotational constants for tHTE-1-(13)C(1) from a B3LYP/cc-pVTZ model with scale factors based on the normal species were in excellent agreement with observations. Similar good agreement was found between predicted and observed ground state rotational constants for the three (13)C(1) isotopologues of cis-hexatriene, as determined from microwave spectroscopy. Equilibrium rotational constants for tHTE and its three (13)C(1) isotopologues, of which two were predicted, were used to find a semiexperimental equilibrium structure for the C(6) backbone of tHTE. This structure shows increased structural effects of π-electron delocalization in comparison with butadiene and some differences from the cis isomer of HTE. Structures predicted with the MP2/cc-pVTZ model are also compared.     

The visible absorption spectrum of OBrO, investigated by Fourier transform spectroscopy

By the utilization of a new laboratory method to synthesize OBrO employing an electric discharge, the visible absorption spectrum of gaseous OBrO has been investigated. Absorption spectra of OBrO have been recorded at 298 K, using a continuous-scan Fourier transform spectrometer at a spectral resolution of 0.8 cm(-1). A detailed vibrational and rotational analysis of the observed transitions has been carried out. The FTS measurements provide experimental evidence that the visible absorption spectrum of OBrO results from the electronic transition C(2A2)-X(2B1). Vibrational constants have been determined for the C(2A2) state (omega(1) = 648.3 +/- 1.9 cm(-1) and omega 2 = 212.8 +/- 1.2 cm(-1)) and for the X(2B1) state (omega 1 = 804.1 +/- 0.8 cm(-1) and omega 2 = 312.2 +/- 0.5 cm(-1)). The vibrational bands (1,0,0), (2,0,0), and (1,1,0) show rotational structure, whereas the other observed bands are unstructured because of strong predissociation. Rotational constants have been determined experimentally for the upper electronic state C(2A2). By modeling the band contours, predissociation lifetimes have been estimated. Further, an estimate for the absorption cross-section of OBrO has been made by assessing the bromine budget within the gas mixture, and atmospheric lifetimes of OBrO have been calculated using a photochemical model.     

Pure rotational spectra of LuF and LuCl

The pure rotational spectra of two isotopic species of LuF and three of LuCl have been measured in the frequency range 5-17 GHz using a cavity pulsed jet Fourier transform microwave spectrometer. The samples were prepared by laser ablation of Lu metal in the presence of SF(6) or Cl(2), and stabilized in supersonic jets of Ar. Spectra of molecules in states having v= 0, 1, and 2 have been measured, to produce rotational constants and centrifugal distortion constants, along with hyperfine constants for all the nuclei. Dunham-type fits for LuCl produced a Born-Oppenheimer breakdown parameter for Cl. Although a theoretical calculation showed that Lu in LuCl should have a significant field shift effect parameter, it could not be determined from the spectrum. Equilibrium internuclear distances, r(e), and dissociation energies have been evaluated for both molecules. The nuclear quadrupole coupling constants are discussed in terms of the molecular electronic structure.     

Rotational spectroscopy and equilibrium structures of S3 and S4

The sulfur molecules thiozone S3 and tetrasulfur S4 have been observed in a supersonic molecular beam in the centimeter-wave band by Fourier transform microwave spectroscopy, and in the millimeter- and submillimeter-wave bands in a low-pressure glow discharge. For S3 over 150 rotational transitions between 10 and 458 GHz were measured, and for S4 a comparable number between 6 and 271 GHz. The spectrum of S3 is reproduced to within the measurement uncertainties by an asymmetric top Hamiltonian with three rotational and 12 centrifugal distortion constants; ten distortion constants, but an additional term to account for very small level shifts caused by interchange tunneling, are required to reproduce to comparable accuracy the spectrum of S4. Empirical equilibrium (r(e)(emp)) structures of S3 and S4 were derived from experimental rotational constants of the normal and sulfur-34 species and vibrational corrections from coupled-cluster theory calculations. Quantum chemical calculations show that interchange tunneling occurs because S4 automerizes through a transition state with D2h symmetry which lies about 500 cm(-1) above the two equivalent C2upsilon minima on the potential energy surface.     

A priori predictions of the rotational constants for HC13N, HC15N, C5O

Ab initio molecular orbital theory is used to estimate the rotational constant for several carbon-chain molecules that are candidates for discovery in interstellar space. These estimated rotational constants can be used in laboratory or astronomical searches for the molecules. The rotational constant for HC13N is estimated to be 0.1073 +/- 0.0002 GHz and its dipole moment 5.4 D. The rotational constant for HC15N is estimated to be 0.0724 GHz, with a somewhat larger uncertainty. The rotational constant of C5O is estimated to be 1.360 +/- 2% GHz and its dipole moment 4.4. D.     

Photofragment Imaging of HNCO Decomposition at 210 nm: the Primary NH(a1￠)+CO(X1§+) Channel

The photodissociation of isocyanic acid (HNCO) on the ˉrst excited singlet state following the excitation at 210 nm was investigated with an ion velocity slice imaging technique by probing the CO fragment. It was found from the (2+1) resonance-enhanced multi-photon ionization (REMPI) spectrum that the CO fragments are rotationally hot with population up to Jmax=50. The velocity imagings of the CO fragments at JCO=30 and 35 indicate that formation of NH(a1￠)+CO(X1§+, v=0) is the predominant dissociation channel at 210 nm. From analysis of the CO fragment translational energy distributions, the NH(a1￠) fragment was observed to be rotationally cold, about half of the available energy was partitioned into the translational motion of fragments after dissociation, and the NH(a1￠)+CO(X1§+) dissociation threshold was determined at 42738§30 cm?1. From analysis of the CO fragment angular distributions, the dissociationanisotropy parameter ˉ was found to be negative, and increasing with the rotational quantum number of the NH fragment, i.e., from ?0.75 at JNH=2-4 to ?0.17 at JNH=11. Impulsive direct and vertical dissociation process of HNCO on the singlet state at 210 nm was conˉrmed experimentally. A classical impact dissociation model was employed to explain the dependence of the ˉ value on the rotational excitation of the NH fragment.     

The Cologne Carbon Cluster experiment: ro-vibrational spectroscopy on C8 and other small carbon clusters

We report on our ongoing efforts in obtaining the IR-spectra of the linear carbon cluster molecules Cn with n=8-13. So far C8, C9, C10, and C13 have been recorded at Cologne. With the exception of C8 all assignments have been secured. For C8 a tentative assignment could be derived with the bandcenter of the sigmau antisymmetric stretching mode located at nu0=2067.9779 cm(-1) and a preliminary rotational constant in the vibrational ground state of B"=0.02068 cm(-1). The measured signal to noise ratio of the ro-vibrational band is fairly weak and thus the lower J ro-vibrational transitions can not be assigned with certainty. As a consequence the band center remains uncertain by 4 J or 0.17 cm(-1). For a more reliable assignment the sensitivity of the system has to be increased by at least one order of magnitude. The envisaged sensitivity increase of our experiment will be discussed along with the intention to perform terahertz observations of the low energetic bending ro-vibrational spectra. These sub-mm wave measurements will be carried out simultaneously with the IR measurements.     

High-resolution infrared spectrum of the nu1 Ba nd of eta5-C5H5NiNO

Gas-phase rotational constants and distortion constants have been determined for the nu1 (v=1) excited vibrational state of cyclopentadienylnickel nitrosyl (C5H5NiNO) using a high-resolution Fourier transform spectrometer system at Kitt Peak, Arizona. The rotationally resolved lines have been measured for the C-H symmetric stretch vibration (nu1=3110 cm(-1)). In the present analysis, over 150 lines have been assigned and fitted using a rigid-rotor Hamiltonian with centrifugal distortion. The vibrational band center, excited-state rotational constants, and distortion constants derived from the measured spectrum for this prolate symmetric-top molecule are nuo=3110.4129(4) cm(-1), A'=0.14328(8) cm(-1), B'=C'=0.041285(1) cm(-1), DJ'=0.078(1) kHz, DJK'=2.23(4) kHz, and DK'=-2.63(2) kHz, respectively. Several different combination differences, with a common upper state, were calculated for different K stacks for the observed spectra, and the consistency of the lower state rotational constants obtained provided further support for the current assignment. The ground-state rotational constant (B') derived from this combination differences analysis agrees with the previously obtained Fourier transform microwave value to within 0.15%. However, ground-state rotational constants, A' and B', have been fixed in the present analysis to avoid correlation effects and to get more accurate results. The new measured parameters are compared with the previously obtained results from Fourier transform microwave and infrared spectroscopy measurements. The C-H vibration stretching frequency and rotational constants were calculated using density functional theory calculations, and these were quite helpful in resolving ambiguities in the fitting procedure and for initial assignments of measured lines.     

Fourier transform microwave spectroscopy of vinyldiacetylene, vinyltriacetylene, and vinylcyanodiacetylene

The rotational spectra of the three carbon chain molecules vinyldiacetylene (hex-1-ene-3,5-diyne, C(6)H(4)), vinyltriacetylene (oct-1-ene-3,5,7-triyne, C8H4), and its cyano analog vinylcyanodiacetylene (1-cyanohex-5-ene-1,3-diyne, C7H3N) have been observed for the first time by Fourier transform microwave spectroscopy of a supersonic molecular beam. The molecules were observed as products of an electrical discharge through selected precursor mixtures: ethylene/diacetylene and vinylacetylene/diacetylene for the pure hydrocarbon molecules and vinylacetylene/cyanoacetylene for vinylcyanodiacetylene. The measurements yield precise sets of rotational constants that compare very well with theoretical constants obtained by quantum chemical calculations at the B3LYP/cc-pVTZ level of theory. Since these three carbon chains are similar in structure and composition to known astronomical molecules and because of their significant polarity, all three are candidates for radio astronomical detection.     

Deactivation of singlet molecular oxygen by organo-selenium compounds exhibiting glutathione peroxidase activity and by sulfur-containing homologs.

The bimolecular rate constants (k) of quenching of molecular singlet oxygen 1O2 (1 delta g) by organo-selenium compounds exhibiting glutathione peroxidase activity and by sulfur analogs have been determined by time resolved phosphorescence detection of 1O2 in CD3OD and C6D6, with no solvent effect. The rate constants of quenching by the Se-containing compounds were found to be approximately one order of magnitude higher than those of the S-containing homologs. A linear correlation was observed between log k and the Hammett constant omega ortho with p = -0.89, the rate constant being higher for molecules with an electron-donating substituent and lower for those with an electron-withdrawing substituent. This observation is consistent with the involvement of a charge transfer complex in the deactivation of singlet oxygen.     

Microwave spectra, geometries, and hyperfine constants of OCAgX (X = F, Cl, Br)

A pulsed jet cavity Fourier transform microwave spectrometer has been used to measure the rotational spectra of OCAgX (X = F, Cl, Br) in the frequency range 5-22 GHz. Metal atoms were generated via laser ablation and were allowed to react with CO and a halide precursor, prior to stabilization of the products within a supersonic jet of argon. These are the first experimental observations of OCAgF and OCAgBr, and the first high resolution spectroscopic study of OCAgCl. All three molecules are linear. Accurately determined rotational constants have been used to evaluate the various internuclear distances, which are found to be consistent with trends established for OCAuX and OCCuX species. The C-O distances are short, and the M-C distances are significantly longer than those in other molecules containing a metal-carbonyl bond. Precise values of centrifugal distortion constants and halogen nuclear quadrupole coupling constants have also been determined. The coupling constants are compared with the results of previous studies of OCCuX and OCAuX and are used to infer trends in the electron distributions of the molecules. Ab initio calculations have been performed and employed to predict the geometries, vibrational frequencies, and Mulliken valence orbital populations of the various species.     

The quantum mechanical calculation of rotational spectra. A comparison of methods for C2H2, HCN, HNC, HCO+, N2H+, CO and N2. Predictions for HCNH+, COH+, HBO, HBNH, and HBF+

Rotational frequencies determined with ab initio molecular orbital theory can play an important role in guiding spectroscopic searches for new molecules and in corroborating the assignment of unidentified lines, from the laboratory and from space. In a systematic study of 22 levels of molecular orbital theory, CISD/6-311G** gave rotational frequencies to an accuracy of +/- 0.4 GHz when an empirical correction is applied to the results for C2H2,HCN, HNC, HCO+, N2H+, CO, and N2. Larger errors can be expected when there are large vibrational effects on the rotational constants, as exemplified by COH+. Predicted J = 0--> 1 rotational frequencies using these methods are 73.9 +/- 0.4 GHz for HCNH+, 78.6 +/- 0.4 GHz for HBO, 65.8 +/- 0.4 GHz for HBNH, and 72.1 +/- 0.4 GHz for HBF+.