Neutral HF Research Articles

Gas-Phase Reactions of Chromium and Chromium Fluoride Cations CrFn+ (n = 0−4) with Water and Nitrous Oxide

The reactions of atomic chromium and chromium fluoride monocations CrFn+ (n = 0−4) with water and nitrous oxide were investigated by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Besides slow condensation, Cr+ is unreactive towards water, whereas all chromium fluoride cations undergo at least one Cr−F bond hydrolysis yielding the corresponding hydroxide concomitant with loss of neutral HF. While hydrolysis is complete for CrF+ and CrF2+, proton transfer and HF losses compete with the hydrolysis of the second Cr−F bond in CrF3+ and CrF4+. Nevertheless, complete Cr−F bond hydrolysis is observed for the latter two cations as well. The results for the ion-molecule reactions with nitrous oxide are as follows: With the exception of CrF3+, which gives CrF3O+ in a good yield, all species investigated were found to be essentially unreactive towards N2O. The ionisation energy of CrF3O has been determined, IE = 11.3 ± 0.2 eV. In addition, limits for the standard heats of formation of several chromium-containing cations have been derived. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Hydrogen bonding. Part. 76. Molecular orbital investigation of the stability of tertiary amine-hydrogen fluoride compounds in aqueous solution

N,N-Dimethyl-1-adamantamine-hydrogen fluoride (2MAAHF) does not dissociate in aqueous solution, while 2MAAHCl and 2MAAHBr readily ionize in H2O. Semi-empirical (AM1-SM2) calculations demonstrate that the enthalpic barrier to dissociation of 2MAAHF in aqueous solution is four times as high as that of 2MAAHCl and three times as high as that of 2MAAHBr. Ab initio calculations (3-21G(*) and 3-61G∗) show that 2MAAHF is a tightly bound molecular complex between neutral HF and neutral amine, with a low dipole moment and little negative charge exposed on fluorine. 2MAAHCl and 2MAAHBr are ion-pairs with an electrostatic hydrogen bond between cation and anion, large dipole moments, and exposed halogen atoms with high electrostatic potential; these factors explain the ease of ionization of 2MAAHCl and 2MAAHBr. Parallel studies on other tertiary amines give similar results.

Metmyoglobin/Fluoride: Effect of Distal Histidine Protonation on the Association and Dissociation Rate Constants

The kinetics of formation and dissociation of the horse metmyoglobin/fluoride complex has been investigated between pH 3.4 and 11. The ionic strength dependence of the reaction has been measured at integral pH values between pH 5 and 10. Hydrofluoric acid, HF, binds to metmyoglobin with a rate constant of (4.7 ± 0.7) × 104M−1s−1. An apparent ionization in metmyoglobin with a pKaof 4.4 ± 0.5 influences the rate of HF binding and is attributed to the distal histidine, His-64. Protonation of His-64 increases the HF binding rate by a factor of 2.6. The fluoride anion, F−, binds to metmyoglobin with a rate constant of (5.6 ± 1.4) × 10−2M−1s−1, about 106times slower than HF. Binding of either HF or F−to hydroxymetmyoglobin cannot be detected. Protonation of the distal histidine facilitates HF dissociation from the metmyoglobin/fluoride complex. HF dissociates with a rate constant of 1.9 ± 0.3 s−1. The fluoride anion dissociates 2000 times more slowly, with a rate constant of (8.7 ± 1.6) × 10−4s−1. The apparent pKafor His-64 ionization in the fluorometmyoglobin complex is 5.7 ± 0.1. The association and dissociation rate constants are relatively independent of ionic strength with secondary kinetic salt effects sufficient to account for the ionic strength variation of both, consistent with the idea that association and dissociation of neutral HF dominate the kinetics of fluoride binding to metmyoglobin.

Potential energy curves for the HF − and CH 3F − anions: a coupled cluster study

Very accurate ab initio calculations using the single-reference (SR) coupled cluster (CC) theory were performed for HF, CH 3F and their negative ions. The CC theory with singly and doubly excited clusters (CCSD approach) and its non-iterative counterpart accounting for connected triexcited components (CCSD(T) approach) were used as primary theoretical methods, and the HF and HF − systems were also investigated using the complete active space self-consistent field (CASSCF) approach. Emphasis was placed on the stability of the negatively charged ions and the potential energy curves for dissociative electron attachment in HF and CH 3F. In agreement with the majority of earlier calculations, the HF − and CH 3F − anions are unstable (i.e. have negative electron affinities) and can only capture an electron when the HF and CF bonds are significantly stretched. It is demonstrated that electron capture by HF and CH 3F correlates quite well with the maxima in the corresponding dipole moment functions. The potential energy curve for HF − is attactive, i.e. without a barrier for electron attachment, representing the HF molecule plus free electron near the equilibrium geometry of HF and crossing the neutral HF curve at the HF internuclear separation of 2.2 bohr in order to reach the H(1 s 2S) + F − (2 6 p 1S) limit (the equilibrium HF bond length is 1.733 bohr). The stretching of the HF bond and the energy required to capture an electron, resulting from CC calculations, are lower than in earlier calculations. The potential energy curve for CF bond breaking in CH 3F − has barriers for the dissociative electron attachment ( ΔE ‡ a ) and associative electron detachment ( ΔE ‡ d ) at the CF distance of approximately 3.7 bohr (the equilibrium CF separation is 2.612 bohr). Both barriers resulting from CC calculations, which are lower than in earlier calculations (CCSD method gives ΔE ‡ a = 1.67 eV , ΔE ‡ d = 0.28 eV ; CCSD(T) results are ΔE ‡ a = 1.56 eV , ΔE ‡ d = 0.20 eV ), are sensitive to many-electron correlation effects. The dissociative electron attachment in methyl fluoride is accompanied by a strong variation of the angle describing the umbrella motion of the methyl group and the appearance of a shallow (approximately 2 kcal mol −1) van der Waals' minimum in the corresponding CH 3F − curve representing the CH 3·F − complex. The implications of our findings for electron transfer phenomena, in particular for the photoinitiated charge transfer processes of the harpooning type, are discussed. The role of diffuse functions in the calculations for the temporary anion states and the role of the basis set and the theoretical approach in the modelling of potentials for electron attachment to a molecule are also briefly discussed.

Effects of Neutral Bases on the Isomerization of Conventional Radical Cations CH3X•+ to Their Distonic Isomers •CH2X+H (X = F, OH, NH2): Proton-Transport Catalysis and Other Mechanisms

High level ab initio theory is used to investigate the effects of the neutral bases HF, H2O, and NH3 on the mechanisms and barriers for isomerization of the conventional radical cations CH3X•+ (X = F, OH, and NH2) to their corresponding distonic isomers •CH2X+H. It is found that the isomerization mechanism is determined largely by the relative proton affinities of the base and the parent radical •CH2X. If the proton affinity of the base is substantially lower than the proton affinity at either C or X of •CH2X, the barrier is lowered but remains positive relative to separated base plus CH3X•+. If the proton affinity of the base lies between that at C and X, the barrier becomes negative and the base successfully catalyzes the isomerization of CH3X•+ to •CH2X+H. In fact, the barrier is found to be negative even in cases where the proton affinity of the base is lower than the lower proton affinity site of •CH2X, provided that this proton affinity difference is not too large. If the proton affinity of the base is higher than that at both C and X, the barrier to rearrangement is lowered even further. However, intermolecular proton transfer from the ion to the base rather than intramolecular proton migration is then the lower energy process. An alternative isomerization mechanism for the [CH3OH/base]•+ systems is also detailed in which the base remains bound to the hydroxyl hydrogen throughout. The barriers for this so-called “spectator” mechanism are found to be higher than those for the interconversion of the isolated conventional and distonic ions. A rationalization based on the nature of the intervening ion−base complexes is presented.

Comparison of single and double excitation coupled cluster and configuration interaction theories: determination of structure and equilibrium propertie

Using our recently implemented closed-shell coupled cluster singles and doubles (CCSD) model, the equilibrium structure and vibrational harmonic frequency of N 2, CO, HF and OH − have been determined. This set ofdiatomics was specially chosen so as to represent the range of bonding characteristics found in small molecules. The CCSD results are compared to analogous configuration interaction (CI) predictions and are found to be most similar to CISDQ or CISDTQ. That is, CI including all singles + doubles + quadruples (CISDQ) or CISDQ+all triple excitations (CISDTQ). The agreement of CCSD with CISDQ/CISDTQ is found to be much better for the singly bonded species with the agreement for neutral HF somewhat better than for anionic OH −. We arrive at two major conclusions. First, higher-order effects not present in the CCSD model are less negligible for multiply bonded species and, to a smaller degree, for anionic species as well. Secondly, even for the two types of molecules discussed above, the CCSD results are closer to the CISDQ/CISDTQ predictions than are CISD values. Since CCSD is less computationally time consuming than CISDQ/CISDTQ, we suggest that the CCSD model is one of the best methods available when highly accurate ab initio predictions for chemical species strongly dominated by a single-determinant wavefunction are desired.

Statistical properties of atomic energy levels. I. Partial level density

General arguments used for studying partial level density in nuclei, when applied to atomic levels, lead one to expect that the electronic states arising from some definite configuration would be distributed in a nearly Gaussian fashion. This feature is investigated for the 3d5 configuration and the 4f6 configuration of electrons in atoms, as well as for the even-parity states in Ta II and the experimentally observed states in the neutral Hf atom. It is found that their distribution is approximately Gaussian, although the departure from normality is not negligible.

Ground Levels and Ionization Potentials for Lanthanide and Actinide Atoms and Ions

Values of the first four ionization potentials of the lanthanides (Z=57–71) and of Hf have been compiled. All except the value for neutral Hf are based on spectroscopic data. The spectroscopic designations of the ground levels of the neutral through triply ionized atoms (Z=57–72) are also tabulated. A similar compilation for the actinides (Z=89–103) lists Sugar's recent values for the first ionization potentials through No (Z=102). Accurate spectroscopic ionization potentials have been determined for only two of the actinide ions (Ac+ and Th3+). The ground-level designations for the neutral through triply ionized actinides are given where they are known or can be predicted with near certainty. A selection of references to the most complete and most recent work on the analyses of the optical spectra of the neutral through triply ionized lanthanide and actinide elements is included.

Mass Spectra of Organometallic Compounds. IV. Some Mononuclear Fluorocarbon Derivatives of Transition Metals

The mass spectra of the following fluorocarbon derivatives of transition metals have been investigated: (a) the perfluoroalkyl derivatives RfMo (CO)3C5H5 ( Rf = CF3 and C3F7); (b) the perfluoroacyl derivatives CF3COM(CO)3C6H5 (M = Mo and W), C3F7COM(CO)3C5H5 (M = Mo and W), and C3F7COFe(CO)2C6H6; (c) the perfluoraryl derivatives RfFe(CO)2C5H5 ( Rf/ = C6F5, p-HC6F4, 3, 4-H2C6F3, and p-CF3C6F4); (d) the hexafluorobutyne derivatives [(CF3)2C]3 MNCCH3 (M = Mo and W); and (e) the bis (trifluoromethyl)tetramethylbicyclo-[2.2,2] octatriene complexes C14H14F6Fe (CO)3 and C5H5CoC14H14F6. Unusual features observed in these mass spectra include the following: (a) Evidence for elimination of the neutral metal fluoride fragments FeF2 and C5H5CoF; (b) evidence for the elimination of neutral HF and CF2 fragments; (c) evidence for the elimination of a bridge in the bicyclo [2,2,2] octatriene derivatives; (d) decarbonylation of RfCOMo (CO)3C5H5 compounds, but not their tungsten analogs, in the mass spectrometer; and (e) evidence for the eliminations of C4F4 and CH3CN fragments in the mass spectra of the [(CF3)2C2]3MNCCH3 complexes.

On a formal theory of resonance reactions: Luciano Fonda, Istituto di Fisica dell'Università, Trieste, Italy; and Istituto Nazionale di Fisica Nucleare, Sottosezione di Trieste, Trieste, Italy

Very accurate ab initio calculations using the single-reference (SR) coupled cluster (CC) theory were performed for HF, CH3F and their negative ions. The CC theory with singly and doubly excited clusters (CCSD approach) and its non-iterative counterpart accounting for connected triexcited components (CCSD(T) approach) were used as primary theoretical methods, and the HF and HF− systems were also investigated using the complete active space self-consistent field (CASSCF) approach. Emphasis was placed on the stability of the negatively charged ions and the potential energy curves for dissociative electron attachment in HF and CH3F. In agreement with the majority of earlier calculations, the HF− and CH3F− anions are unstable (i.e. have negative electron affinities) and can only capture an electron when the HF and CF bonds are significantly stretched. It is demonstrated that electron capture by HF and CH3F correlates quite well with the maxima in the corresponding dipole moment functions. The potential energy curve for HF− is attactive, i.e. without a barrier for electron attachment, representing the HF molecule plus free electron near the equilibrium geometry of HF and crossing the neutral HF curve at the HF internuclear separation of 2.2 bohr in order to reach the H(1s2S) + F− (26p1S) limit (the equilibrium HF bond length is 1.733 bohr). The stretching of the HF bond and the energy required to capture an electron, resulting from CC calculations, are lower than in earlier calculations. The potential energy curve for CF bond breaking in CH3F− has barriers for the dissociative electron attachment (ΔE‡a) and associative electron detachment (ΔE‡d) at the CF distance of approximately 3.7 bohr (the equilibrium CF separation is 2.612 bohr). Both barriers resulting from CC calculations, which are lower than in earlier calculations (CCSD method gives ΔE‡a = 1.67 eV, ΔE‡d = 0.28 eV; CCSD(T) results are ΔE‡a = 1.56 eV, ΔE‡d = 0.20 eV), are sensitive to many-electron correlation effects. The dissociative electron attachment in methyl fluoride is accompanied by a strong variation of the angle describing the umbrella motion of the methyl group and the appearance of a shallow (approximately 2 kcal mol−1) van der Waals' minimum in the corresponding CH3F− curve representing the CH3·F− complex. The implications of our findings for electron transfer phenomena, in particular for the photoinitiated charge transfer processes of the harpooning type, are discussed. The role of diffuse functions in the calculations for the temporary anion states and the role of the basis set and the theoretical approach in the modelling of potentials for electron attachment to a molecule are also briefly discussed.

Second-order Z-dependent theory of many-electron atoms: David Layzer, Zdeněk Horák, Margaret N. Lewis, and Daniel P. Thompson, Harvard College Observatory, Cambridge, Massachusetts

Very accurate ab initio calculations using the single-reference (SR) coupled cluster (CC) theory were performed for HF, CH3F and their negative ions. The CC theory with singly and doubly excited clusters (CCSD approach) and its non-iterative counterpart accounting for connected triexcited components (CCSD(T) approach) were used as primary theoretical methods, and the HF and HF− systems were also investigated using the complete active space self-consistent field (CASSCF) approach. Emphasis was placed on the stability of the negatively charged ions and the potential energy curves for dissociative electron attachment in HF and CH3F. In agreement with the majority of earlier calculations, the HF− and CH3F− anions are unstable (i.e. have negative electron affinities) and can only capture an electron when the HF and CF bonds are significantly stretched. It is demonstrated that electron capture by HF and CH3F correlates quite well with the maxima in the corresponding dipole moment functions. The potential energy curve for HF− is attactive, i.e. without a barrier for electron attachment, representing the HF molecule plus free electron near the equilibrium geometry of HF and crossing the neutral HF curve at the HF internuclear separation of 2.2 bohr in order to reach the H(1s2S) + F− (26p1S) limit (the equilibrium HF bond length is 1.733 bohr). The stretching of the HF bond and the energy required to capture an electron, resulting from CC calculations, are lower than in earlier calculations. The potential energy curve for CF bond breaking in CH3F− has barriers for the dissociative electron attachment (ΔE‡a) and associative electron detachment (ΔE‡d) at the CF distance of approximately 3.7 bohr (the equilibrium CF separation is 2.612 bohr). Both barriers resulting from CC calculations, which are lower than in earlier calculations (CCSD method gives ΔE‡a = 1.67 eV, ΔE‡d = 0.28 eV; CCSD(T) results are ΔE‡a = 1.56 eV, ΔE‡d = 0.20 eV), are sensitive to many-electron correlation effects. The dissociative electron attachment in methyl fluoride is accompanied by a strong variation of the angle describing the umbrella motion of the methyl group and the appearance of a shallow (approximately 2 kcal mol−1) van der Waals' minimum in the corresponding CH3F− curve representing the CH3·F− complex. The implications of our findings for electron transfer phenomena, in particular for the photoinitiated charge transfer processes of the harpooning type, are discussed. The role of diffuse functions in the calculations for the temporary anion states and the role of the basis set and the theoretical approach in the modelling of potentials for electron attachment to a molecule are also briefly discussed.

The ultra-violet spectra of HF and DF

Successful analyses have been m ade of th e ultra-violet spectra excited by electric discharges in m ixtures of helium with HF or DF. The spectra, which consist of large numbers of irregularly spaced lines, are shown to arise from transitions V 1 Σ + - X 1 Σ + in neutral HF and DF. State X is the ground state: state V , the lowest stable excited singlet state, probably correlates with H + + F - . The potential minimum of state V lies at 10·5 eV above that of the ground state. The internuclear distance in this state —in agreement with theoretical predictions — is large, more than twice that in the ground state, so that the bands observed consist of transitions from low vibrational levels in V to high vibrational levels in X . In HF, the observed levels are 9 ≤ v " ≤ 19: predissociation by rotation is observed in the levels 10 ≤ v " ≤ 19. The last vibrational state which may have a few stable rotational levels is v " = 20, so that, by combining the present observations with those of the vibration-rotation bands, there is now available information about the vibrational and rotational levels over the greater part of their range of stable existence in the ground state. The dissociation energy, D" 0 , of HF is found to be 5·86 ±0·01 eV.