Laser Ablation Molecular Beam Research Articles

Formation of interstellar cyanoacetamide: a rotational and computational study.

Cyanoacetamide is a -CN bearing molecule that is also an amide derivative target molecule in the interstellar medium. The aim of our investigation is to analyze the feasibility of a plausible formation process of protonated cyanoacetamide under interstellar conditions and to provide direct experimental frequencies of the ground vibrational state of the neutral form in the microwave region in order to enable its eventual identification in the interstellar medium. We used high-level theoretical computations to study the formation process of protonated cyanoacetamide. Furthermore, we employed a high-resolution laser-ablation molecular beam Fourier transform spectroscopic technique to measure the frequencies of the neutral form. We report the first rotational characterization of cyanoacetamide, and a precise set of the relevant rotational spectroscopic constants have been determined as a first step to identifying the molecule in the interstellar medium. We fully explored the potential energy surface to study a gas-phase reaction on the formation process of protonated cyanoacetamide. We found that an exothermic process with no net activation barrier is initiated by the high-energy isomer of protonated hydroxylamine, which leads to protonated cyanoacetamide.

Alkaline and Alkaline-earth Cyanoacetylides: A Combined Theoretical and Rotational Spectroscopic Investigation.

The metallic cyanoacetylides LiC3N, NaC3N, MgC3N and CaC3N have been investigated by combined spectroscopy measurements and theoretical calculations. The theoretical calculations predict for the four species that the linear isomer with formula MCCCN (M= Li, Na, Mg and Ca) is the most stable one. We used the laser ablation molecular beam Fourier transform microwave spectroscopy to synthesize these species by the reaction of metal vapors, produced by laser ablation, and the 3-bromo-2-propynenitrile (BrCCCN). The pure rotational spectra were observed by Fourier transform microwave spectroscopy in the 2-18 GHz frequency region only for LiCCCN and NaCCCN, while no spectral signatures for MgCCCN and CaCCCN could be detected. Finally, we have searched for LiCCCN and NaCCCN species towards the carbon-rich evolved star IRC + 10216 but only upper limits to their abundances have been obtained.

Ionization energies and structures of small lanthanum oxide clusters (La2O3)n.LaO (n = 1–3)

Small lanthanum oxide clusters were produced in a laser-ablation molecular beam setup and studied by mass spectrometry and laser threshold photoionization spectroscopy. Mass spectrometry investigations of neutral clusters revealed certain stoichiometries, (La2O3)n and (La2O3)n.LaO (n = 1–6) were stable. For cation clusters, similar stoichiometric preference was reported by previous authors. The stoichiometric preference of the oxides was not affected by oxygen concentrations. Ionization thresholds and vertical ionization energies (VIEs) were measured for the (La2O3)n.LaO (n = 1–3) neutral clusters from the photoionization efficiency curves. The energetically low-lying structural isomers for these clusters along with their adiabatic and vertical ionization energies were computed from the density functional theory using B3LYP and PW91GGA exchange correlation functionals. Favorable agreement between the computed and experimental ionization energies allows assignments of the ground state structures for these clusters. For La3O4, a C3v-like structure has been found as the ground state built on La-O-La bonds with their usual oxidation states of +3 for La and −2 for O atoms. For larger clusters La5O7 and La7 O10, however, a trend towards forming more compact structures with few metal and O sites having higher coordination numbers have been found as the ground states and are suggested to be present in the molecular beam.

Low-Energy Photoelectron Imaging Spectroscopy of Lan(benzene) (n = 1 and 2).

Lan(benzene) (n = 1 and 2) are formed in a pulsed laser-ablation molecular beam source and characterized by low-energy photoelectron imaging spectroscopy. The photoelectron spectrum of La2(benzene) displays a strong origin band, a short metal-ligand stretching progression, and a weak ring deformation band. Four isomers are considered for La2(benzene), and the preferred structure is an inverse sandwich with two La atoms residing on the opposite sides of the benzene ring. The ground electronic state of the inverse sandwich is 1A1g (D3d) with (5dxy,x2-y2 + π*)46s2 electron configuration. Ionization removes a La-based 6s electron and yields a 2A1g ion. The spectrum of La(benzene) is similar to the zero-electron kinetic energy spectrum reported previously by our group, although the spectral line width is somewhat broader. The measurement of the photoelectron angular distribution of La(benzene) confirms that the ejected electron has largely a p wave character. The metal-ligand bonding of La2(benzene) is considerably stronger than that of La(benzene) due to the threefold binding of each La atom in the dilanthanum species and the twofold binding in the monolanthanum complex.

The role of amino acid side chains in stabilizing dipeptides: the laser ablation Fourier transform microwave spectrum of Ac-Val-NH2.

The steric effects imposed by the isopropyl group of valine in the conformational stabilization of the capped dipeptide N-acetyl-l-valinamide (Ac-Val-NH2) have been studied by laser ablation molecular beam Fourier transform microwave (LA-MB-FTMW) spectroscopy. The rotational and quadrupole coupling constants of the two 14N nuclei determined in this work show that this dipeptide exists as a mixture of C7 and C5 conformers in the supersonic expansion. The conformers are stabilized by a C[double bond, length as m-dash]OH-N intramolecular hydrogen bond closing a seven- or a five-membered ring, respectively. The observation of both conformers is in good agreement with previous results on the related dipeptides containing different residues, confirming that the polarity/non-polarity of the side chains of the amino acid is responsible for the conformational locking/unlocking. The voluminous isopropyl group is not able to prevent the less stable C5 conformer from forming but it destabilizes the C[double bond, length as m-dash]OH-N interaction.

Laser spectroscopy of the [17.9]4 – X3Φ4 and [15.6]Ω – X3Φ4 transitions of iridium monochloride (IrCl)

Abstract Laser Induced Fluorescence (LIF) spectra of two electronic transitions, [17.9]4 – X 3 Φ 4 and [15.6]Ω – X 3 Φ 4 , of IrCl have been obtained in a laser-ablation molecular beam source, at low resolution using a pulsed dye laser, and at high resolution using a single mode ring dye laser. The 193 Ir 35 Cl– 193 Ir 37 Cl isotope shifts of the band heads in the low resolution spectra established the vibrational assignment of the 0-0, 1-0, 2-0 and 0-1 bands of the [17.9]4 – X 3 Φ 4 transition and the 0-0 and 1-0 bands of the [15.6]Ω – X 3 Φ 4 transition. High resolution spectra (linewidth ∼ 0.006 cm −1 FWHM) were obtained for the 0-0, 1-0 and 2-0 bands of the more intense [17.9]4 – X 3 Φ 4 transition. Rotational structure in the three bands of both 193 Ir 35 Cl and 191 Ir 35 Cl isotopologues was resolved and analyzed. The higher J rotational lines are observed to split into closely spaced doublets resulting from quadrupole hyperfine structure caused by the I = 3/2 nuclear spin in 191 Ir and 193 Ir. Examination of the spectra shows that the doublet separation of ∼0.008 cm −1 is similar to that observed in IrF (Linton et al., 2011) indicating that the quadrupole parameters, eQq 0 , are similar in IrF and IrCl.

Laser spectroscopy of jet-cooled NiF: Application of Hougen’s approximate model for the low-lying electronic states

Abstract We have taken laser-induced fluorescence spectra of jet-cooled nickel monofluoride formed in a laser-ablation molecular beam source. Dispersed-fluorescence spectroscopy confirms predictions by Hougen (2011) that the parity assignments of levels in the Ω = 1 / 2 state 1570 cm −1 above the ground state should be reversed from those given in Krouti et al. (2002). The quality of the high-resolution spectra was sufficient to measure the [22.9]1.5–X1.5 band for five isotopologues of nickel and the [22.9]1.5–[0.25]0.5 band for molecules containing 58 Ni and 60 Ni. The spectral line positions for each isotopologue were fit to the Hamiltonian model given by Hougen, which was extended to allow for calculation of the parity-splitting parameter in the ground state.

Generation and structural characterization of aluminum cyanoacetylide.

Combined spectroscopy measurements and theoretical calculations bring to light a first investigation of a metallic cyanoacetylide, AlC3N, using laser ablation molecular beam Fourier transform microwave spectroscopy. This molecule was synthesized in a supersonic expansion by the reaction of aluminum vapour with C3N, produced from solid aluminum rods and BrCCCN in a newly constructed ablation-heating nozzle device. A set of accurate rotational and (27)Al and (14)N nuclear quadrupole coupling constants have been determined from the analysis of the rotational spectrum and compared with those predicted in a high-level ab initio study, conducting to the assignment of the observed species to linear AlCCCN. We have searched for this species towards the carbon-rich evolved star IRC + 10216 but only an upper limit to its abundance has been obtained.

Rotational and hyperfine structure in the [17.6]2.5–X2.5 and [23.3]2.5–X2.5 transitions of iridium monoxide, IrO

Abstract Laser induced fluorescence spectra of two electronic transitions, [17.6]2.5–X2.5 and [23.3]2.5–X2.5, of IrO have been obtained at high resolution by using a single mode ring dye laser to excite IrO molecules in a laser-ablation molecular beam source. The 191 IrO– 193 IrO isotope shifts in the rotational lines, observed at a linewidth of ∼0.006 cm −1 FWHM, established the vibrational assignment of the [23.3]2.5–X2.5 band as 1–0 and confirmed the previous 0–0 assignment of the [17.6]2.5–X2.5 band. The higher J rotational lines of both transitions are observed to split into closely spaced doublets resulting from quadrupole hyperfine structure caused by the I = 3/2 nuclear spin in 191 Ir and 193 Ir. Analysis of the spectra shows that the hyperfine structure is concentrated in the two excited states with quadrupole coupling constants, eQq 0 , of −0.0463(15) cm −1 and −0.0278(18) cm −1 for the [17.6]2.5 and [23.3]2.5 states respectively.

All Five Forms of Cytosine Revealed in the Gas Phase

Give me five! All five tautomers and conformers of cytosine were characterized in the gas phase by laser ablation molecular beam Fourier transform microwave spectroscopy. The spectra were assigned unambiguously on the basis of the hyperfine structure due to the three 14N nuclei (see picture; N blue, O red). The relative energies of the identified species were estimated from the relative intensities of the spectra.

Determination of the Ground Electronic State in Transition Metal Halides: ZrF

The spectroscopy of the ZrF radical, produced by a laser ablation-molecular beam experimental setup, has been investigated for the first time using a two-color two-photon (1 + 1') REMPI scheme and time-of-flight (TOF) mass spectrometry detection. The region of intense bands 400-470 nm has been studied, based upon the first spectroscopic observations of the isovalent ZrCl radical by Carroll and Daly. The overall spectrum observed is complex. However, simultaneous and individual ion detection of the five naturally occurring isotopologues of ZrF has provided a crucial means of identifying band origins and characterization via the isotopic shift, δ(iso), of the numerous vibronic transitions recorded. Hence, five (0-0) transitions, of which only two were free of overlap with other transitions, have been identified. The most intense (0-0) transition at 23113 cm(-1) presented an unambiguously characteristic RQP rotational structure. From rotational contour simulations of the observed spectra, the nature of the ground electronic state is found to be unambiguously of (2)Δ symmetry, leading to the assignment of this band as (0-0) (2)Δ ← X(2)Δ at 23113 cm(-1). A set of transitions (1-0) (2)Δ ← X(2)Δ at 22105 cm(-1) and (2-0) (2)Φ ← X(2)Δ at 22944 cm(-1) involving the X(2)Δ state has also been identified and analyzed. Furthermore, a second series of transitions with lesser intensity has also been related to the long-lived metastable (4)Σ(-) state: (3-0) (4)Π(-1/2) ← (4)Σ(-) at 21801 cm(-1), (2-0) (4)Π(-1/2) ← (4)Σ(-) at 21285 cm(-1) and (2-0) (4)Σ(-) ← (4)Σ(-) at 23568 cm(-1). These spectroscopic assignments are supported by MRCI ab initio calculations, performed using the MOLPRO quantum chemistry package, and show that the low-lying excited states of the ZrF radical are the (4)Σ(-) and (4)Φ states lying at 2383 and 4179 cm(-1) respectively above the ground X(2)Δ state. The difference in the nature of ground state and ordering of the first electronic states with TiF (X(4)Φ)(2-4) and ZrCl,(5) respectively, is examined in terms of the ligand field theory (LFT)(7) applied to diatomic molecules. These results give a precise description of the electronic structure of the low lying electronic states of the ZrF transition metal radical.

Jet-Cooled Rotational Spectrum of Laser-Ablated Phenylalanine

The rotational spectrum of neutral phenylalanine has been recorded for the first time using laser-ablation molecular-beam Fourier transform microwave spectroscopy (LA-MB-FTMW). Two conformers stabilized by conjugative O-H···N and N-H···π hydrogen bond interactions have been conclusively identified on the basis of experimental values of rotational and (14)N nuclear quadrupole coupling constants. The nonobservation of the rotational spectra of the other low-energy conformers has been attributed to the photofragmentation that takes place in the laser ablation process. Nuclear quadrupole coupling interactions have been used directly to determine the orientation of the amino group and to identify and experimentally characterize the N-H···π interactions.

Nuclear quadrupole coupling interactions in the rotational spectrum of tryptamine

Abstract Four conformers of tryptamine have been detected in a supersonic expansion and characterized by laser ablation molecular beam Fourier transform microwave spectroscopy LA-MB-FTMW in the 5–10 GHz frequency range. The quadrupole hyperfine structure originated by two 14 N nuclei has been completely resolved for all conformers and used for their unambiguous identification. Nuclear quadrupole coupling constants of the nitrogen atom of the side chain have been used to determine the orientation of the amino group involved in N–H⋯π interactions: to the π electronic system of the pyrrole unit in the Gauche-Pyrrole conformers (GPy) or to the phenyl unit in the Gauche-Phenyl ones.

The two conformers of acetanilide unraveled using LA-MB-FTMW spectroscopy

Abstract Acetanilide has been investigated by laser ablation molecular beam Fourier transform microwave LA-MB-FTMW spectroscopy. The rotational spectrum of both trans and cis conformers have been analyzed to determine the rotational and 14N quadrupole coupling the constants. The spectrum of the less abundant cis conformer has been assigned for the first time. The doublets observed for this conformer have been interpreted in terms of the tunneling motion between two equivalent non-planar conformations through a small barrier in which the acetamide group and phenyl ring plane are perpendicular. The results are compared with those of the related formanilide.

Hydrogen bonding and structure of uracil–water and thymine–water complexes

The complexes uracil-water and thymine-water have been investigated using laser ablation molecular-beam Fourier transform microwave (LA-MBFTMW) spectroscopy. Only one conformer of each complex has been observed. The spectra of the parent and several isotopically enriched species with (15)N at uracil moiety and D or (18)O at water have been measured. For uracil-water the spectra are characterized by the hyperfine structure of two (14)N nuclei. For thymine-water only the parent and (18)O-water enriched species have been observed. Their spectra are characterized by the hyperfine structure of two (14)N nuclei with an additional doubling due to the A and E states of the torsion of the methyl group. The barrier to internal rotation of the methyl group has been determined from the analysis of the spectrum. Investigation of the structure of the adducts from the rotational constants of the different isotopologues shows that the observed conformers correspond to the most stable forms in which water closes a cycle with the nucleic acid bases forming N-H(NB)...O(w) and H(W)...O=C(2) hydrogen bonds.

A comparative theoretical study of the reactivities of the Al+ and Cu+ ions toward methylamine and dimethylamine

A very recent laser ablation-molecular beam experiment shows that an Al+ ion can react with a single methylamine (MA, CH3NH2) or dimethylamine (DMA, (CH3)2NH) molecule to form a 1:1 ion–molecule complex Al+[CH3NH2] or Al+[(CH3)2NH)], whereas a dehydrogenated complex ion Cu+[CH3N] or Cu+[C2H5N] is detected, respectively, in the similar reaction for a Cu+ ion. Here, we show a comparative density functional theory study for the reactivities of the Al+ and Cu+ ions toward MA and DMA to reveal the intrinsic mechanism. It is found that the interactions of the Al+ ion with MA and DMA are mostly electrostatic, leading to the direct ion–molecule complexes, Al+NH2CH3 and Al+NH( CH3)2, in contrast to the non-negligible covalent character in the corresponding Cu+-containing complexes, Cu+NH2CH3 and Cu+NH( CH3)2. The general dehydrogenation mechanism for MA and DMA promoted by the Cu+ ion has been shown, and the preponderant structures contributing to the mass spectra of the product ions Cu+[CH3N] and Cu+[C2H5N] are rationalized as Cu+NHCH2 and Cu+N( CH2)( CH3). The presumed dehydrogenation reactions are also discussed for the Al+-containing systems. However, the involved barriers are found to be too high to be overcome at low energy conditions. These results have rationalized all the experimental observations well. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Conformational Study of Taurine in the Gas Phase

The conformational preferences of the amino sulfonic acid taurine (NH(2)-CH(2)-CH(2)-SO(3)H) have been investigated in the gas phase by laser ablation molecular beam Fourier transform microwave spectroscopy (LA-MB-FTMW) in the 6-14 GHz frequency range. One conformer has been observed, and its rotational, centrifugal distortion, and hyperfine quadrupole coupling constants have been determined from the analysis of its rotational spectrum. Comparison of the experimental constants with those calculated theoretically identifies the detected conformer unambiguously. The observed conformer of taurine is stabilized by an intramolecular hydrogen bond O-H...N between the hydrogen of the sulfonic acid group and the nitrogen atom of the amino group.

Seven conformers of l-threonine in the gas phase: a LA-MB-FTMW study

Rotational spectroscopy in combination with molecular beams and laser ablation (laser-ablation molecular-beam Fourier transform microwave (LA-MB-FTMW) spectroscopy) has proved to be successful in characterizing the conformers of natural amino acids. The procedure usually followed to assign and identify the different conformers of an amino acid from the rotational spectrum is described through the study of the natural amino acid L-threonine. The solid sample of L-threonine was vaporized by laser pulses, diluted in Ne and supersonically expanded between the mirrors of a Fabry-Pérot resonator where it was spectroscopically probed by microwave radiation. The rotational and nuclear quadrupole coupling constants extracted from the analysis of the rotational spectrum are directly compared with those predicted by ab initio methods to achieve the conclusive identification of seven different conformers. A complex hydrogen bonding network arises as a consequence of the polar side chain of threonine.

Observation of two new conformers of neutral proline

Using laser-ablation molecular-beam Fourier transform microwave (LA-MB-FTMW) spectroscopy two new conformers of proline have been identified together with the two previously observed (Angew. Chem. Int. Ed., 2002, 41, 4673).

The Rotational Spectrum of TiO2

The rotational spectrum of titanium dioxide (TiO2) has been detected by laser-ablation molecular beam Fourier transform microwave spectroscopy. Thirteen b-type rotational transitions up to J = 9 and Ka = 4 were measured for the most abundant isotopic species 48TiO2 in the frequency range 7-42 GHz with accuracies of 1-10 kHz, allowing for the precise determination of rotational and centrifugal distortion parameters. In addition, eight and six rotational transitions of the rare isotopic species 46TiO2 and 50TiO2, respectively, have been recorded in the same frequency range. From the derived spectroscopic parameters, rest frequencies of TiO2 can now be calculated to better than 1 km s−1 in equivalent radial velocity up to fairly high J and Ka at millimeter wavelengths, enabling radioastronomical searches for this stable, highly polar transition metal dioxide in space. Preliminary results of two searches for TiO2 in space are presented.