Ar Complexes Research Articles

Theoretical study of the complex between formic acid and argon

Quantum chemical calculations on the structure, energetics and vibrational spectra of the van der Waals complex between formic acid (FA) and argon are presented. Both conformers of FA were considered in the calculations and for both forms a planar structure with an almost linear interaction path between argon and the OH-tail of the molecular is the most stable complex configuration. The computed interaction energies at the CCSD(T)/6-311++G(2d,2p),6-311+G(3df)[Ar]//MP2(full)/6-311++G(2d,2p),6-311+G(3df)[Ar] level of theory are −1.50 and −2.01 kJ mol −1 for the trans-HCOOH⋯Ar and cis-HCOOH⋯Ar complexes, respectively. Calculations reveal three other local minima for both complexed conformers of which a local minimum structure featuring interaction between argon and the carbonyl group of the molecule is characterised for the first time. The intra- and intermolecular stretching modes were found to be most affected by BSSE-correction applied during the optimisation of the complex equilibrium structure. The intramolecular stretching modes were noted to decrease and the intermolecular stretching modes to increase compared to calculations without the BSSE-correction.

Theoretical study of the external heavy atom effect on phosphorescence of free-base porphin molecule

The radiative lifetime of phosphorescence of free-base porphin (H 2P) molecule and its complexes with noble-gas atoms are calculated by time-dependent density functions theory (TD DFT) with quadratic response functions for account of spin–orbit coupling and electric dipole activity. The complexes with Ne, Ar, Kr, and Xe are used to simulate the external heavy atom (EHA) effect on phosphorescence of the H 2P molecule in the corresponding noble gas matrices. The B3LYP functional and small basis set (3–21G) are used throughout the study and comparison of all complexes but other basis sets are also utilized to support the chosen approach. A slow radiative rate constant of free-base porphin phosphorescence (about 10 −3 s −1) is obtained with all basis sets being in the order of magnitude agreement with experimental estimations. A strong enhancement of the H 2P phosphorescence rate (by 360 times) is calculated for Xe complex; while for Ne, Ar, and Kr complexes, the enhancement is equal to 1.1, 1.3, and 10.3 times, respectively. In these complexes, the noble gas atom is disposed at 3.6 Å above the center of the porphin ring. In spite of shortcomings of the chosen simple model, the TD DFT calculations explain the most important features of the EHA effect on phosphorescence of free-base porphin. Calculations of the hyperfine coupling tensors for all magnetic nuclei in the lowest triplet state of H 2P molecule and its complexes with noble-gas atoms indicate an appreciable penetration of the spin density to the EHA region. This can be connected with the enhancement of spin–orbit coupling in the H 2P molecule.

Intracluster stereochemistry in van der Waals complexes: steric effects in ultraviolet photodissociation of state-selected Ar-HOD/H2O.

High-resolution IR-UV multiple resonance methods are employed to elucidate the photodissociation dynamics of quantum state-selected Ar-HOD and Ar-H(2)O van der Waals clusters. A single mode pulsed OPO operating in the region of the OH second overtone is used to prepare individual rovibrational states that are selectively photodissociated at specific excimer wavelengths. Subsequent fluorescence excitation of the resulting OH (OD) fragments yields dynamical information on the photofragmentation event and any resulting intracluster collisions. This technique is used to characterize spectroscopically the Pi(1(01)), nu(OH)=3<--Sigma(0(00)), v(OH)=0 overtone band of the Ar-HOD complex with an origin at 10648.27 cm(-1). The effects of Ar complexation on the dissociation dynamics are inferred by comparison of the OD photofragment quantum state distributions resulting from dissociation of single rovibrational states of the complex with those from isolated HOD photodissociation. The important role played by the initial internal state of the complex is demonstrated by comparison of the current Ar-HOD data with previously published results for the Ar-H(2)O Sigma(0(00))[03(-)> state. We interpret the dramatic differences in the dynamics of the two systems as manifestations of the nodal structure of the vibrational state in the parent complex and the way in which it governs the collision probability between the Ar atom and the escaping photofragments.

The C3-bending levels of the C3-Ar complex studied by optical spectroscopy and ab initio calculation.

The A-X electronic transition of C3-Ar, near 405 nm, has been studied by both laser-induced fluorescence and wavelength-resolved emission techniques. Emission spectra have been recorded from 14 vibrational levels of the A state of C3-Ar; these spectra consist of progressions in the ground state v2 and v4 vibrations (the in- and out-of-plane C3-bending motions, respectively). With increasing bending excitation, these ground state levels shift progressively downwards compared to those of free C3, indicating that the van der Waals complexes are becoming more tightly bound. The level structure of the two vibrations of C3-Ar has been fitted to a perturbed harmonic oscillator model, where the potential function has the form V = V1r cos theta + V2r2 cos 2theta (r is the amplitude of the C3-bending motion and theta gives the orientation of the rare gas atom relative to the plane of the bent C3 molecule). Ab initio calculations have been carried out for C3-Ar at the coupled-cluster singles, doubles (and triples)/correlation consistent polarization valence quadruple-zeta level. They predict that the C3-Ar complex is nearly T shaped at equilibrium, and that as the C3 molecule bends away from the linear configuration, the preferred orientation is "arrow" shaped. From the results of the best fit to the model and the emission spectral intensities, the relative orientation of the out-of-plane pi electron of the A-state complex and the Ar atom has been estimated. No bands of the Ar complex were found near the C3, A-X, (0,0) band, consistent with the fact that the A 1Piu, upsilon = 0 level of free C3 is strongly perturbed by triplet levels. In the excitation spectra of the Ar complex, the bands with upsilonb' > 0 show redshifts of about 16-36 cm(-1) compared to those of free C3, indicating that the A-state complex in these levels is more tightly bonded than the X-state complex.

Theoretical study of the complex between formic acid and argon

Quantum chemical calculations on the structure, energetics and vibrational spectra of the van der Waals complex between formic acid (FA) and argon are presented. Both conformers of FA were considered in the calculations and for both forms a planar structure with an almost linear interaction path between argon and the OH-tail of the molecular is the most stable complex configuration. The computed interaction energies at the CCSD(T)/6-311++G(2d,2p),6-311+G(3df)[Ar]//MP2(full)/6-311++G(2d,2p),6-311+G(3df)[Ar] level of theory are −1.50 and −2.01 kJ mol −1 for the trans-HCOOH⋯Ar and cis-HCOOH⋯Ar complexes, respectively. Calculations reveal three other local minima for both complexed conformers of which a local minimum structure featuring interaction between argon and the carbonyl group of the molecule is characterised for the first time. The intra- and intermolecular stretching modes were found to be most affected by BSSE-correction applied during the optimisation of the complex equilibrium structure. The intramolecular stretching modes were noted to decrease and the intermolecular stretching modes to increase compared to calculations without the BSSE-correction.

A positron beam study of vacancy–impurity complexes in inert gas ion-implanted silicon

Silicon (100) wafers have been implanted with 1×1015 He+ or Ar+ ions/cm2 and the vacancy (V)–He or –Ar complexes induced have been investigated with positron lifetime and coincidence Doppler broadening measurements using a positron beam. He+ ion implantation into Si gives rise to formation of V2He complexes, which are transformed into VxHey (x–y=1) complexes by annealing at 300°C. The amorphous phase induced by Ar irradiation of Si is transformed into a crystalline phase around 700°C. The crystallization accompanies formation of V–Ar complexes with six vacant sites, and these complexes are found to be stable above 800°C. These results have proved that positrons are very useful probes to study the chemical state of vacancy-inert gas atoms complexes.

Density functional theory analysis of CaRg n+ complexes: (Rg=He, Ne, Ar; n=1–4)

CaRg n + (Rg=He, Ne, Ar) complexes with n=1–4, are investigated by performing using the B3LYP/6-311+G (3df) density functional theory calculations. The CaHe n + ( n=1–4) complexes are found to be stable. In the case of CaNe n + and CaAr n +, stable structures and stationary point were found only for n=1 and 2. For n=3 in the C 3 V and the D 3 h point group as well as for n=4 in the T d (tetrahedral) point group a saddle point (imaginary frequency) is observed and global minimum could be obtained along the potential energy surface.

Generation of “unstable” doubly charged metal ion complexes in a laser vaporization cluster source

Doubly charged metal ion complexes of the form M 2+(L) n are generated using a laser vaporization cluster source in conjunction with a time-of-flight mass spectrometer. Contrary to expectations, a variety of doubly charged species are produced with this source, including many so-called “unstable” or “metastable” ions in which the metal has a second ionization potential greater than the first ionization potential of the ligand or solvent. The species identified include Mg 2+(CO 2) n , Mg 2+(H 2O), Mg 2+(Ar) n , Co 2+(Ar) n , Co 2+(H 2O), Si 2+(Ar) n and Ti 2+(CO 2) n . This is apparently the first observation by any means of Co 2+(Ar) n , Ti 2+(CO 2) n and Si 2+(Ar) n . Of the complexes studied, only the “stable” species Mg 2+(Ar) n have been generated previously by laser vaporization. The conditions necessary for the production of these ions are investigated and possible mechanisms for their growth are suggested. Charge-transfer photodissociation is observed for Co 2+(Ar) complexes.

High precision iron isotope measurements of meteoritic material by cold plasma ICP-MS

The first cold plasma ICP-MS (inductively coupled plasma mass spectrometer) Fe isotope study is described. Application of this technique to the analyses of Fe isotopes in a number of meteorites is also reported. The measurement technique relies on reduced temperature operation of the ICP source to eliminate pervasive molecular interferences from Ar complexes associated with conventional ICP-MS. Instrumental mass bias corrections are performed by sample-standard bracketing and using Cu as an external mass bias drift monitor. Repeated measurements of a terrestrial basalt reference sample indicate an external reproducibility of ± 0.06 ‰ for δ 56Fe and ± 0.25 ‰ for δ 58Fe (1 σ). The measured iron isotopic compositions of various bulk meteorites, including irons, chondrites and pallasites are identical, within error, to the composition of our terrestrial basalt reference sample suggesting that iron mass fractionation during planet formation and differentiation was non-existent. Iron isotope compositions measured for eight chondrules from the unequilibrated ordinary chondrite Tieschitz range from −0.5 ‰ < δ 56Fe chondrules < 0.0 ‰ relative to the terrestrial/meteorite average. Mechanisms for fractionating iron in these chondrules are discussed.

Free jet rotational spectrum of propylene oxide–krypton and modelling and ab initio calculations for propylene oxide–rare gas dimersElectronic supplementary information (ESI) available: Tables S1 and S2: Experimental transition frequencies of PRO⋯84Kr and PRO⋯86Kr complexes. See http://www.rsc.org/suppdata/cp/b3/b300386h/

The rotational spectra of propylene oxide-84Kr and propylene oxide-86Kr have been measured by free jet millimetre-wave absorption spectroscopy. The preferred experimentally derived conformation of the complex is supported by results obtained with a simple distributed polarisability model and by ab initio calculations. The dissociation energy of the complex has been estimated, from the DJ centrifugal distortion constant, to be ca. 3.7 kJ mol−1, which compares with 3.3 kJ mol−1 from the model. Modelling and ab initio calculations have also been performed for the related propylene oxide–Ne and propylene oxide–Ar complexes, and led to a re-assignment of their absolute energy minima. The results obtained for the three propylene oxide–rare gas atom dimers are compared.

IR spectra of para-substituted phenol +–Ar cations: effect of halogenation on the intermolecular potential and O–H bond strength

IR spectra of isolated Ar complexes of the phenol, para-fluorophenol, and para-chlorophenol cations (Ph +–Ar, p-FPh +–Ar, p-ClPh +–Ar) are analyzed in the vicinity of the O–H stretch fundamental, ν 1. Two isomers of each dimer are produced in the electron impact ion source and identified by their characteristic ν 1 frequency, namely the H-bound global minimum and the less stable π-bound local minimum. The analysis of the ν 1 frequencies indicates that the strength of the O–H bond in p-XPh + increases by halogen substitution in the order X=H<F<Cl. Quantum chemical calculations support the interpretation of the experimental data.

A Comprehensive Computational Study on OCH+-Rg (Rg = He, Ne, Ar, Kr, Xe) Complexes

The equilibrium geometries, harmonic vibrational frenquencies, and the dissociation energies of the OCH+-Rg (Rg = He, Ne, Ar, Kr, and Xe) complexes were calculated at the DFT, MP2, MP4, CCSD, and CCSD(T) levels of theory. In the lighter OCH+-Rg (Rg = He, Ne, Ar) rare gas complexes, the DFT and MP4 methods tend to produce longer Rg-H+ distance than the CCSD(T) level value, and the CCSD-calculated Rg-H+ bond lengths are slightly shorter. DFT method is not reliable to study weak interaction in the OCH+-He and OCH+-Ne complexes. A qualitative result can be obtained for OCH+-Ar complex by using the DFT method; however, a higher-level method using a larger basis set is required for the quantitative predictions. For heavier atom (Kr, Xe)-containing complexes, only the CCSD method predicted longer Rg-H+ distance than that obtained at the CCSD(T) level. The DFT method can be applied to obtain the semiquantitative results. The relativistic effects are expected to have minor effect on the geometrical parameters, the H+-C stretching mode, and the dissociation energy. However, the dissociation energies are sensitive to the quality of the basis set. The nature of interaction between the OCH+ ion and Rg atoms was also analyzed in terms of the interaction energy components.

Noble gas-actinide compounds: evidence for the formation of distinct CUO(Ar)(4-n)(Xe)(n) and CUO(Ar)(4-n)(Kr)(n) (n = 1, 2, 3, 4) complexes.

Laser-ablated U atoms react with CO in excess argon to produce CUO, which gives rise to 852.5 and 804.3 cm-1 infrared absorptions for the triplet state CUO(Ar)n complex in solid argon at 7 K. Relativistic density functional calculations show that the CUO(Ar) complex is stable and that up to four or five argon atoms can complex to CUO. When 1-3% Xe is added to the argon/CO reagent mixture, strong absorptions appear at 848.0 and 801.3 cm-1 and dominate new four-band progressions, which increase on annealing to 35-50 K as Xe replaces Ar in the intimate coordination sphere. Analogous spectra are obtained with 1-2% Kr added. This work provides evidence for eight distinct CUO(Ng)n(Ar)4-n (Ng = Kr, Xe, n = 1, 2, 3, 4) complexes and the first characterization of neutral complexes involving four noble-gas atoms on one metal center.

Intermolecular vibration and internal rotation of a methyl group in acetanilide·Ar: a ZEKE photoelectron spectroscopy study

Two-color REMPI and ZEKE photoelectron spectra are presented for the trans-acetanilide·Ar complex. Methyl rotor features are prominent in the cluster spectra and appear with unperturbed frequencies and intensities compared to acetanilide, indicating that the argon atom exerts little influence on the methyl rotor from its van der Waals binding position above the aromatic ring. Analysis of the spectra allowed the determination of the S1 and D0 barrier heights (50 and 320 cm−1), and the S0 and S1 0a–1e splitting (3.9 and 2.0 cm−1).

Ab initio ground state potential energy surfaces for Rg–Br2 (Rg=He, Ne, Ar) complexes

High-level ab initio molecular electronic structure calculations are performed for Rg–Br2 (Rg=He, Ne, Ar) complexes at CCSD(T) (coupled cluster using single and double excitations with a noniterative perturbation treatment of triple excitations) level of theory. Specific augmented correlation consistent basis sets are used for each noble atom (Rg), supplemented with an additional set of bond functions. Effective-core potentials (ECPs), augmented with diffusion (sp) and polarization (3df ) functions, have been employed for the bromine atoms. For all complexes, the CCSD(T) potential energy surfaces (PESs) show double-minimum topology, with wells at both linear and T-shaped configurations; the linear minimum is found to be deeper than the T-shaped one. Vibrational corrections are taken into account for all the complexes and their effects in the stability of the linear and T-shaped conformers are examined. For each complex and each configuration (linear and T-shaped), Re equilibrium intermolecular distances, De and D0 dissociation energies, are evaluated and compared with previous theoretical and/or experimental results.

An investigation of the quadrupole coupling of two N nuclei in the complex pyrimidine–Ar by molecular beam Fourier transform microwave spectroscopy

The quadrupole coupling hyperfine structure of two equivalent 14N nuclei in the rotational spectrum of pyrimidine–Ar complex has been investigated in the range of 6–18 GHz using molecular beam Fourier transform microwave spectroscopy. From the analysis of the hyperfine structure the elements χ aa , χ bb , χ cc and χ bc of the quadrupole coupling tensors in the principal inertial axis system of the complex have been determined. A comparison of the quadrupole constants of the complex with those of bare pyrimidine indicates that these constants are consistent with the structure of the complex previously reported, with the Ar atom tilted ca. 5° towards the two N atoms. This configuration is supported by ab initio calculations.

Structural Determinants of the Na+/H+Exchanger Regulatory Factor Interaction with the β2Adrenergic and Platelet-derived Growth Factor Receptors

The Na(+)/H(+) exchanger regulatory factor (NHERF) binds through its PDZ1 domain to the carboxyl-terminal sequences NDSLL and EDSFL of the beta(2) adrenergic receptor (beta(2)AR) and platelet-derived growth factor receptor, respectively, and plays a critical role in the membrane localization and physiological regulation of these receptors. The crystal structures of the human NHERF PDZ1 domain bound to the sequences NDSLL and EDSFL have been determined at 1.9- and 2.2-A resolution, respectively. The beta(2)AR and platelet-derived growth factor receptor ligands insert into the PDZ1 binding pocket by a beta-sheet augmentation process and are stabilized by largely similar networks of hydrogen bonds and hydrophobic contacts. In the PDZ1-beta(2)AR complex, the side chain of asparagine at position -4 in the beta(2)AR peptide forms two additional hydrogen bonds with Gly(30) of PDZ1, which contribute to the higher affinity of this interaction. Remarkably, both complexes are further stabilized by hydrophobic interactions involving the side chains of the penultimate amino acids of the peptide ligands, whereas the PDZ1 residues Asn(22) and Glu(43) undergo conformational changes to accommodate these side chains. These results provide structural insights into the mechanisms by which different side chains at the position -1 of peptide ligands interact with PDZ domains and contribute to the affinity of the PDZ-ligand interaction.

Collision-induced intramolecular energy flow and C–H bond dissociation in excited toluene

The collision-induced intramolecular energy flow and C–H bond dissociation in toluene have been studied using classical dynamics procedures. The molecule initially contains high amounts of vibrational excitation in the methyl C–H stretch and the nearby benzene ring C–H stretch and it is in interaction with Ar. The two excited C–H stretches are coupled to each other through two C–C stretching, two H–C–C bending and one C–C–C bending modes, all of which are initially in the ground state. At 300 K, the energy lost by the excited molecule upon collision is not large and it increases slowly with increasing total vibrational energy content between 10 000 and 40 000 cm−1. Above the energy content of 40 000 cm−1, energy loss increases rapidly. Near 65 000 cm−1 energy loss takes a maximum value of about 1000 cm−1. The temperature dependence of energy loss is weak between 200 and 400 K. When the energy content is sufficiently high, either or both C–H bonds can dissociate, producing free radicals, C6H5CH2, C6H4CH3, or C6H4CH2. The ring C–H dissociation occurs almost entirely in a direct-mode mechanism on a subpicosecond time scale. Nearly half of methyl group C–H dissociation events occur on a subpicosecond time scale and the rest through a complex-mode collision in which bond dissociation occurs several picoseconds after the initial impact. In the complex-mode collision, Ar binds to the radical forming a weakly bound benzyl⋯Ar complex. In both dissociative and nondissociative events, intramolecular energy flow is efficient, taking place upon the initial impact on a subpicosecond time scale.

The most stable conformer of the propylene oxide–argon complex

The free jet millimeter-wave absorption spectra of the propylene oxide–Ar complex is reported. The Ar atom lies, with respect to the plane of the COC ring, on the same side of the methyl group ( syn conformer). This is in contrast with the results of MP2/6-311++G ∗∗ calculations, which give the anti conformer slightly more stable. The ab initio geometry of the syn species is, however, in agreement with the experimental findings. The dissociation energy has been estimated, from the D J centrifugal distortion constant, to be about 3 kJ/mol.

Infrared photodissociation spectroscopy of benzene–Ne,Ar complex cations

The infrared (IR) absorption spectrum of the jet-cooled benzene cation complexed with Ne has been recorded throughout the 275–1600 cm−1 and the 2800–3200 cm−1 ranges via IR-laser induced vibrational dissociation spectroscopy. Measuring the spectrum of the complex ion rather than the spectrum of the bare benzene cation, leads to the appearance of weak Van der Waals sidebands. In addition, subtle effects of the symmetry lowering in the complex, giving rise to additional lines in the spectrum, have been observed for the benzene–Ar complex. To obtain a more detailed picture of the vibrational structure of the benzene cation in the region up to 1500 cm−1, IR absorption spectra of benzene complex cations prepared in selected low-lying vibrationally excited levels have been recorded.