HX Complexes Research Articles

Experimental and Computational Study of HXeY···HX Complexes (X, Y = Cl and Br): An Example of Exceptionally Large Complexation Effect

The complexes of xenon hydrides HXeY (Y = Cl and Br) with hydrogen halides HX (X = Cl and Br) have been studied both computationally and experimentally in a xenon matrix. The experiments revealed three new complexes: HXeBr...HBr, HXeBr...HCl, and HXeCl...HCl. The experimental assignments were done on the basis of the strong H-Xe stretching absorption of HXeY (Y = Cl and Br) molecules and supported by theoretical results. We experimentally obtained monomer-to-complex blue-shifts of this vibrational mode for all the studied systems (up to approximately 150 cm (-1)). The electronic structure calculations revealed three local structures for each HNgY...HX complexes and their computed interaction energies varied between -460 and -2800 cm (-1). The computational estimates of the vibrational shifts were in agreement with the experimental values. We also found possible experimental absorption belonging to HXeBr...(HBr) 2 trimer and its vibrational shift (+245 cm (-1)) is similar to the computational estimate of a cyclic ternary complex (+252 cm (-1)).

A theoretical study of hydrogen complexes of the X [sbnd]H-π type between propyne and HF, HCL or HCN

The present manuscript reports a systematic investigation of the basis set dependence of some properties of hydrogen-bonded (π type) complexes formed by propyne and a HX molecule, where X = F, Cl and CN. The calculations have been performed at Hartree–Fock, MP2 and B3LYP levels. Geometries, H-bond energies and vibrational have been considered. The more pronounced effects on the structural parameters of the isolated molecules, as a result of complexation, are verified on R C C and HX bond lengths. As compared to double-ζ (6-31G **), triple-ζ (6-311G **) basis set leads to an increase of R C C bond distance, at all three computational levels. In the case where diffuse functions are added to both hydrogen and ‘heavy’ atoms, the effect is more pronounced. The propyne–HX structural parameters are quite similar to the corresponding parameters of acetylene–HX complexes, at all levels. The largest difference is obtained for hydrogen bond distance, R H, with a smaller value for propyne–HX complex, indicating a stronger bond. Concerning the electronic properties, the results yield the following ordering for H-bond energies, Δ E: propyne⋯HF > propyne⋯HCl > propyne⋯HCN. It is also important to point out that the inclusion of BSSE and zero-point energies (ZPE) corrections cause significant changes on Δ E. The smaller effect of ZPE is obtained for propyne⋯HCN at HF/6-311++G ** level, while the greatest difference is obtained at MP2/6-31G ** level for propyne⋯HF system. Concerning the IR vibrational it was obtained that larger shift can be associated with stronger hydrogen bonds. The more pronounced effect on the normal modes of the isolated molecule after the complexation is obtained for H X stretching frequency, which is shifted downward.

Electronic structure and vibrational analysis of AHA⋯HX complexes

Electronic structures of the binary complexes of acetohydroxamic acid (AHA) and hydrogen halides, HX (X = F, Cl, Br) have been investigated using the second order perturbation theory. In the lowest energy structure of AHA⋯HF complex, hydrogen fluoride acts as a proton-donor with carbonyl oxygen and simultaneously as a proton-acceptor with the hydroxyl group. For chloro- and bromo-substituted derivatives, however, the lowest minimum possesses hydrogen-bonded interactions with the carbonyl oxygen in addition to those from the methyl proton of AHA. Frequency shifts of NH and CN stretching vibrations enable one to distinguish different conformers of AHA⋯HX complexes.

Spectroscopy and dynamics of hydride radical van der Waals complexes

Spectroscopic and theoretical studies of hydride radical–rare gas atom complexes (Rg–HX) are reviewed. This family of van der Waals molecules is of interest as they can be used to explore the characteristics of the long-range forces associated with open-shell species. Orbitally degenerate states of HX radicals have an electronic anisotropy that results in van der Waals interactions that are qualitatively different from those exhibited by the corresponding closed-shell systems. Rg–HX complexes, where X is a first- or second-row p-block element, reveal systematic trends where the anisotropic components of the physical interactions are determined by the electronic orbital configuration. Radicals in Σ, Π and Δ states with singlet, doublet and triplet spin multiplicities have been examined. When Rg = He, Ne or Ar the interaction potential energy surfaces can be predicted using high-level ab initio methods. Theoretical studies have established the methods and basis sets that are capable of providing an accurate description of the long-range forces for open-shell molecules. Clusters consisting of an HX molecule with multiple rare gas atoms are model systems for studies of solvated radicals. Potential energy surfaces derived from the binary clusters are being used to construct approximate potentials for Rg n –HX clusters. The equilibrium structures and vibrational dynamics predicted for these systems show that solvated radicals exhibit unique properties. Contents PAGE 1. Introduction 376 2. Experimental methods 377 3. Theoretical considerations 378 4. Binary complexes 384 4.1. BH–Ar and AlH–Ar 384 4.2. CH–Rg complexes (Rg = He, Ne and Ar) 386 4.3. NH–Rg complexes (Rg = He, Ne and Ar) 397 4.4. OH–Rg and SH–Rg complexes 407 5. Complexes of HX radicals with multiple Rg atoms 413 6. Conclusions and future directions 417 Acknowledgements 418 References 418

Infrared matrix isolation studies of the acetohydroxamic acid complexes with HF and HCl

Argon matrix infrared spectra of the complexes formed between acetohydroxamic acid (CH 3CONHOH) and hydrogen halides (HF, HCl) have been recorded. The experimental results indicate formation of a strong binary complex in which the hydrogen halide molecule acts as a proton donor towards the carbonyl group of acetohydroxamic acid. The H–X stretches and several perturbed CH 3CONHOH vibrations were identified for the two HX complexes; for the HF complex the two librational HF modes were also observed.

Isotope effects in ethylene–HX complexes (X=F, Cl or Br)

Ab initio calculations were performed at the MP2/6-311++G(2d,2p) level of theory to obtain optimized geometries, dipole moments, binding energies and harmonic vibrational frequencies for the symmetrical T-shaped structures of XH⋯π bonded complexes involving ethylene (as the proton-acceptor) and the hydrogen halide molecules HF, HCl and HBr (as the proton donors). The relative stabilities of the D-containing isotopomers C 2H 4⋯DX and C 2H 2HD⋯HX were also determined from their zero-point energies. It was found that the D-bonded isotopomer (C 2H 4⋯DX) was more thermodynamically stable than the H-bonded isotopomer (C 2H 2HD⋯HX) for HF, HCl and HBr-containing complexes, previous studies on the weakly bound T-shaped complexes C 2H 2⋯HX also found that the D-bonded species was more stable than the H-bonded species.

Matrix Infrared Spectra and ab Initio Calculations of the Formohydroxamic Acid Complexes with HF and HCl

Argon matrix infrared spectra of the complexes formed between formohydroxamic acid (HCONHOH) or its isotopic analogues (HCONDOD, HCO15NHOH) and hydrogen halides (HF, HCl) have been recorded. The experimental results indicate formation of a very strong binary complex in which the hydrogen halide molecule acts as a proton donor toward the carbonyl group of formohydroxamic acid. The H−X stretches and several perturbed HCONHOH, HCONDOD, and HCO15NHOH vibrations were identified for the two HX complexes; for the HF complex two librational HF modes were also observed. Theoretical studies of the structure and spectral characteristics of the HCONHOH···HF and HCONHOH···HCl complexes were carried out on the electron correlation level with the 6-311++G(2d,2p) basis set. The calculated vibrational frequencies for the complexes present in matrixes are in good agreement with the experimental data. The calculations demonstrate also the stability of the other three binary complexes. In the cyclic structure the HX molecule acts as a proton donor for the CO group and a proton acceptor for the NOH group, and in the two less stable structures the HX is attached to the oxygen or nitrogen atom of the NOH group.

An application of chemometric techniques to analyze the effects of wave function modifications on the binding energies of hydrogen-bonded complexes

Factorial design and principal component models are used to determine how ab initio H-bond energies depend on characteristics of the molecular orbital wave functions of HCN–HX linear complexes and acetylene–HX T-shaped complexes, with X=F, NC, Cl, CN and CCH. The results obtained for the two sets of complexes show that factorial design and principal component analyses complement each other. The H-bond energies of the HCN–HX linear complexes are affected mostly by adding polarization functions and by MP2 treatment, which have opposite signs. When polarization functions are introduced in the basis set the calculated H-bond energies decrease by ≈4 kJ·mol −1. In contrast, when the level of calculation is changed from Hartree–Fock to MP2, the energies are increased by nearly the same amount on average. The principal component analysis shows that for the HCN–HX linear complexes the ab initio results are grouped into four well-separated classes: (I) HF calculations with polarization functions (HF/6- nG** and HF/6- n++G**), with n=31 or 311; (II) MP2 calculations without polarization functions (MP2/6- nG and MP2/6- n++G); (III) MP2 calculations including polarization functions (MP2/6- nG** and MP2/6- n++G**) and (IV) HF results without polarization functions (HF/6- nG and HF/6- n++G). The ab initio results of the group (III) are those that show a better agreement with the available experimental values. The change from the Hartree–Fock to the MP2 level is the most significant effect for the acetylene–HX complexes, increasing the calculated H-bond energy values by about 4.5 kJ·mol −1 on average. Here the ab initio results can be essentially grouped into two classes: HF or MP2 calculations. The H-bond energies calculated from the factorial models for the two sets of complexes are found to deviate only 6.5% from the full ab initio values.

The influence of water molecules on the proton position in H3N–HX (X=F, Cl, Br) complexes

Quantum chemical calculations have been performed to investigate the molecular structures and proton positions of the first few water clusters of H3N–HX (X=F,Cl,Br) hydrogen-bonded complexes. The minimum energy structures were found using the density functional methods at the modified B3-LYP/6-311++G(d,p) level. It is shown that for the H3N–HBr complex only one water molecule is necessary for a proton transfer to occur. In the case of H3N–HCl and H3N–HF systems, the presence in the cluster of two and three water molecules, respectively, induces proton transfer to the ammonia subunit.

Quantum chemical study of hydrogen-bonded CH 2CO…HX and CH 2CCH 2…HX (X = Cl, F) complexes

An ab initio quantum chemical study, largely at the 6–311 + + G ∗ ∗/ MP2 = full level of theory has been carried out on the 1:1 and 2:1 complexes of HX (X = Cl, F) with ketene and are compared with the corresponding isoelectronic complexes of H 2CCCH 2 and HX. Three possible modes of the hydrogen bond between HX and ketene via the dipole-induced dipole (X-H…C, X-H…O) and the weak X-H…π interactions were considered. The binding energy has been calculated after giving careful consideration to basis set superposition effects and zero-point vibrational energy effects. The electron correlation correction to the binding energy has also been computed. Weak X-H…π hydrogen bonds are perhaps possible with nonpolar CC and CC pi-clouds.

Optical properties of nanoscale, one-dimensional silicon grating structures

We report a detailed study of nanostructure fabrication and optical characterization of sub-μm-period, one-dimensional, Si grating structures. Nanoscale wall width structures were fabricated by combining laser interferometric lithography with anisotropic wet-chemical etching (KOH) and thermal oxidation. Structure wall widths were characterized by Raman scattering (RS) and scanning electron microscopy. Salient features of the RS measurements as a function of wall widths from ∼100 to 10 nm were: (a) large cross-section enhancements, ∼100×, for linewidths ∼50 nm; (b) asymmetric line shapes with tails extending to smaller Raman shifts for linewidths ∼20 nm; and (c) splitting of the bulk Raman mode, again to lower Raman shifts, for linewidths ∼10 nm. For room temperature photoluminescence (PL) measurements, the grating structures were excited at 257 nm. PL measurements are reported for oxidized and unoxidized grating structures with peaks varying between 380 and 700 nm. PL was only observed for Si structures with dimensions less than about 10 nm. PL intensities and spectral line shapes varied significantly as a result of surface modification treatments such as high temperature anneal in a N2 atmosphere, immersion in boiling H2O, and long-term exposure to ambient air. The measurements indicate a strong correlation of the visible PL with crystal size (∼5–10 nm); however, it remains unclear if the mechanism responsible is quantum confinement, passivation of the surface by Si:Hx complexes, or optically active surface states.

Determination of D00 (Ca…HBr) from full and half collision studies. Van der Waals and charge transfer contributions to the formation of Ca…HX (X = Cl, Br) complexes

The dissociation energy of the weakly bound complex Ca…HBr in it ground state D 0 0 (Ca…HBr), has been determined using energy balance arguments. Byu combining collisional information from molecular beam studies on the Ca( 3 P) + HBr → CaBr(A) 2II ) + h reaction and spectroscopic infomration from the emission of the product ogf the reaction Ca + HBr → Ca…HBr → hv CaBr (A 2II) + H, we find D 0 0 (Ca…HBr) ≤ 270 ± 50 meV. The measured bond enery is duiscussed as due to the combined effect of interactions of van der Waals and ionic type. An empirical description of both these contributions accounts for the results obtained for Ca…HBr, as well as for those previously reported for Ca…HCl.

Investigations of hydrogen‐bonded systems: Local density approximation and gradient corrections

AbstractThe applicability of the local density approximation (LDA) and of corresponding gradient corrections (for the exchange and correlation energy) for the treatment of the hydrogen bond is investigated. As test systems, we consider the water dimer and the H2O…︁HX complexes (X = F, Cl, Br): Using an LCAO scheme, their equilibrium geometries and interaction energies are ćalculated and compared with experimental data and with other calculations. We obtain that the LDA gives the geometries in qualitative agreement with other data, whereas the energies are overestimated. The use of the gradient corrections (GC) according to Becke and Perdew leads to a significant improvement of the geometry, and especially of the interaction energies. The calculations indicate further that LDA + GC should also be able to describe weaker intermolecular interactions than the usual hydrogen bond. Finally, a short discussion of the charge distribution and the dipole moments of the H2O…︁HX complexes is performed. © 1994 John Wiley & Sons, Inc.

Matrix isolation investigation of the vibrational spectroscopy and photochemistry of complexes of HCl and HBr with nitric oxide. ab initio study of the NO:HCl complex structure

The matrix isolation technique has been used to isolate and characterize the complexes two hydrogen halides (HCl and HBr) and the nitric oxide (monomer and cis dimer). Identification of the complexes was performed through the observation of the HX and (NO) x (x=1,2) submolecule stretching modes. Ab initio calculation shows that the most stable structure ClH:NO is linear with a 0.002 Å shortening of the NO bond distance and an increase of the HCl bond length with an identical amount. Photochemistry of the identified complexes was also studied. The photoproduct of both NO:HX and (NO) 2 HX complexes is XNO (X=Cl, Br) upon UV irradiation. In the case of the (NO) 2:HX complex the XNO molecule is complexed to HNO in the matrix cage.

Non-linearity of the cooperativity effects in hydrogen bond complexes involving hydrogen halides in solid argon

Literature data are available for B⋯HaX⋯HbX complexes in solid argon. In this low temperature medium, the frequency shifts of the HaX (Δνa) and HbX (Δνb) stretching vibrations can be measured and the data can be compared with the frequency shifts of the B⋯HX (Δνs) complexes. Over a very broad domain, extending from the very weak complexes to the strong ones, the Δνa and Δνb values are not linearly related to Δνb but can be expressed by a second degree polynomial of the form a + bΔνs = cΔνs2, the contribution of the quadratic term being higher for Δνb than for Δνa. The librational frequencies follow an equation of the same type. The data are compared with the Kleeberg cooperativity factors Δνa/Δνs or Δνb/Δνs. These two factors decrease with the proton affinity of the base and it is shown that for a proton affinity of about 140 kcal mol−1, the proton affinities of the base and of the F atom of the inner HaF molecule are equal. The key factors determining the cooperativities are the proton donor and proton acceptor ability of the inner HaX submolecule and for a very limited number of systems the cooperativities seem to depend on the protofelicity defined by Lohr (J. Phys. Chem., 88 (1984) 3607).

Infrared spectroscopy of CO2–D(H)Br: Molecular structure and its reliability

A high resolution rovibrational absorption spectrum of the weakly bonded CO2–DBr complex has been recorded in the 2350 cm−1 region by exciting the CO2 asymmetric stretch vibration with a tunable diode laser. The CO2–DBr band origin associated with this mode is 2348.2710 cm−1, red-shifted by 0.87 cm−1 from uncomplexed CO2. The position of the hydrogen atom is determined from differences in moments-of-inertia between CO2–DBr and CO2–HBr, i.e., by using the Kraitchman method. From this, we conclude that ground state CO2–H(D)Br has an average geometry that is planar and inertially T-shaped, with essentially parallel HBr and CO2 axes. Average values of intermolecular parameters are: Rcm=3.58 Å, θBrCO=79.8°, and θHBrC=93.1°. The validity of using the Kraitchman method, which was designed for use with rigid molecules, with a floppy complex like CO2–HBr is discussed. The experimental structure is corroborated qualitatively by results from Mo/ller–Plesset second-order perturbation calculations, corrected for basis set superposition errors. The theoretical equilibrium geometry for the inertially T-shaped complex is planar with structural parameters: RCBr=3.62 Å, θBrCO=89°, and θHBrC=86°. A number of cuts on the four dimensional intermolecular potential surface confirm large zero-point amplitudes, which are known to be characteristic of such systems, and these cuts are used to estimate tunneling splittings. Tunneling is shown to occur by out-of-plane rotation of the H atom, in accord with the experimental observations of Rice et al. There is no significant in-plane tunneling. A quasilinear hingelike isomer (OCO–HBr) with ROH=2.35 Å at equilibrium is calculated to be as stable as the T-shaped complex; however, this species has yet to be observed experimentally. Photoinitiated reactions in CO2–HX complexes are discussed.

Hydrogen bonding between vinylacetylene and HF: the role of steric effects in the geometry of vinylacetylene…HX complexes

The rotational spectrum of a hydrogen-bonded dimer between vinylacetylene (H 2CCHCCH) and HF has been obtained with a pulsed-nozzle Fourier-transform microwave spectrometer. Analysis of the derived spectroscopic constants shows that the HF molecule attaches preferentially to the triple bond in vinylacetylene, in a T-type configuration close to the CC bisector, at a perpendicular distance R CC…F of 3.072 Å. The r o structure of the dimer is planar and a limit of 9° is placed on the equilibrium departure from planarity by consideration of the average r * structure. This is in contrast to an out-of-plane angle of 34° in vinylacetylene…HCl; the difference is rationalised in terms of changes in steric interaction between vinylacetylene and the HX proton donor.

The mechanism of 2-methylbutene-2 hydrohalogenation in solid phase

Complexes and reactions of 2-methylbutene-2 with hydrohalogen (HCl, HBr) have been studied in solid phase at 80–150 K. It has been found that 2-methylbutene-2 forms with HX complexes of 1:1 and 1:2 composition. Hydrohalogenation proceeds via the rearrangement of complex 2HX·C 5H 10 into complex of the addition product with HX. Kinetic equation depends on the reagents ratio. In excess of HX (1< HX:C 5H 10< 10) reaction can be described by the first order kinetic equation. If the ratio HX:C 5H 10 is more than 10, reaction is described by polychronous kinetic law. The effective activation energy of solid phase hydrohalogenation does not exceed 20 kJ mole . The molecular mechanism of hydrohalogenation in solid phase has been proposed.

An ab initio SCF-MO study of the weakly hydrogen bonded complexes CO…HF, CO…HCl, OC…HF and OC…HCl

Abstract Large-scale gaussian orbital SCF-MO results are reported for the title molecules. R e (X…H) and D e (X…H) values are given, and in either case the complexes OC…HF and DC…HCl are more stable than the corresponding CO…HX complexes. The changes on electronic distribution on complex formation are discussed in terms of Mulliken population indices.

The rotational and hyperfine spectrum of Ar–HF

The radio frequency and microwave spectrum of Ar–HF has been remeasured by molecular beam electric resonance spectroscopy. Analysis of the data yields a revised value for the HF nulear spin–spin constant S, which in turn provides information about the vibrationally averaged geometry of the complex. Rotational and hyperfine constants and the dipole moment have also been measured for Ar–DF: The average structures and estimated harmonic force constants are shown to be consistent with those observed for other Ar–HX complexes. Comparison of the two hyperfine constants for Ar–DF indicates that the average charge distribution around the deuterium is only weakly perturbed by the argon atom.