José M. Riveros

Microwave Spectrum and Structure of Nitric Acid

The microwave spectra of five further isotopic species of nitric acid, H18ONO2, cis‐HON18OO, trans‐HONO18O, cis‐HO15N18OO, and D15NO3, have been investigated. The data have been used to refine the previous structural determination of Millen and Morton. In the calculation of structure, particular consideration has been given to the use of ground‐state moments of inertia in Kraitchmans equations for near‐oblate planar molecules. The structure was calculated using several different methods, including the double‐substitution method due to Pierce, in order to substantiate the following structural parameters: O–H=0.964, (H)O–N=1.406, cis–N–O=1.211, trans‐N–O=1.199 A, ∠HON=102°9′, ∠cis‐ONO=115°53′, ∠trans‐ONO=113°51′, and O···O (NO2 group)=2.184 A.This determination clearly demonstrates a 2° tilt of the NO2 group away from the hydrogen atom. The NO2 parameters are close to those found in a number of related molecules, but there is some evidence that the cis‐NO bond in HNO3 is slightly longer than the trans‐NO b...

Gibbs energy of solvation of organic ions in aqueous and dimethyl sulfoxide solutions

The Gibbs energy of solvation of several ions in water and dimethyl sulfoxide (DMSO) solutions was obtained through the use of thermodynamic equations relating ΔGsolv* of the ion with gas phase basicity, pKa, ΔGsolv* of neutral species and the Gibbs energy of solvation of the proton. We have used the most accurate and recent values for these properties, and this report provides 56 Gibbs energy of solvation values in aqueous solution and 30 in DMSO solution. Our results support the general view that anions are much better solvated in aqueous solution than in DMSO. An important example is the hydroxide ion for which the Gibbs energy of transfer from water to DMSO is 26 kcal mol−1. The majority of anions have a Gibbs energy of transfer in the range 10 to 15 kcal mol−1. In the case of cations, DMSO has a larger solvation ability but the difference in the Gibbs energy of solvation between water and DMSO is not greater than 5 kcal mol−1. The present data can be very useful for the development of continuum solvation models.

New values for the absolute solvation free energy of univalent ions in aqueous solution

Abstract The absolute solvation free energy of 30 univalent ions, mainly organic species, has been calculated from experimental and theoretical data on proton affinities, aqueous acidity constants, solvation free energy of neutral species, and the new value for the absolute solvation free energy of the proton determined by Tissandier et al. [J. Phys. Chem. A 102 (1998) 7787]. Our new values reveal considerable differences with previous compilations, and should be taken into consideration for comparison with liquid simulation results and in the development of implicit solvation models.

Gas-phase solvated negative ions

The association of negative ions to neutral molecules has been an active field of research in gas-phase ion chemistry since 1970. For anions, which correspond to the conjugate base of well-known acids in solution, these species are often referred to as gas-phase solvated anions. The ability to form anions bound to a progressive number of neutrals has been particularly explored as a means to understand ion solvation and to bridge the behavior between isolated molecules in the gas-phase and in the bulk. Different mass spectrometric techniques have been used to generate these ions and to determine their stability, structure, and spectroscopic characteristics. The thermodynamic quantities associated with the binding of these negative ions with protic and aprotic solvent-type molecules have been determined over the years. For mono-solvated anions, binding energies to a series of homologous neutrals scale with the gas-phase acidity of the solvent molecule. Although hydrogen bonding is very important for anions with strongly localized charges, ion-dipole interactions become almost as important for large anions like Br− and I−. Theoretical calculations and spectroscopic measurements have provided some unusual insight into the structure of these species, and the present view is that ions like Cl− and Br− do not display a shell-like structure even when attached to a large number of solvent molecules (∼10). Considerable progress has also been made in the study of differences in the reactivity of gas-phase “nude” anions compared with “solvated” anions. The present review outlines some of the most important developments along these different lines.

A Theoretical Analysis of the Free-Energy Profile of the Different Pathways in the Alkaline Hydrolysis of Methyl Formate In Aqueous Solution

The free-energy profile for the different reaction pathways available to the hydroxide ion and methyl formate in aqueous solution is reported for the first time. The theoretical analysis was carried out by using the cluster-continuum method recently proposed by us for calculating the free energy of solvation of ions. Unlike the gas-phase reaction, our results are consistent with the fact that the reaction occurs mainly by nucleophilic attack of the hydroxide on the carbonyl carbon to yield a tetrahedral intermediate (B(AC)2 mechanism). However, an additional pathway, in which the hydroxide ion acts as a general base and a water molecule coordinated to this ion acts as the nucleophile, is also predicted to be important. The relative importance of these pathways is calculated to be 87 % and 13 %, respectively. The tetrahedral intermediate of the hydrolysis reaction has an estimated lifetime of 10 nanoseconds, and its conjugate acid has a pK(a) of 8.8. This tetrahedral intermediate is predicted to proceed to products by two pathways: elimination of methoxide ion (84 %) and by water catalyzed elimination of methanol (16 %). The less common reaction pathway, which involves attack of the hydroxide ion on the formyl hydrogen (decarbonylation mechanism) and leads to water, carbon monoxide, and methanol, is calculated to be only 3 kcal mol(-1) less favorable than the B(AC)2 mechanism. By comparison, direct attack of the hydroxide ion on the methyl group (B(AL)2 or S(N)2 mechanism) leading to an acyl-oxygen bond cleavage has a very high free energy of activation and is not expected to be important. The theoretically observed activation free energy at 298.15 K is calculated to be 15.5 kcal mol(-1), in excellent agreement with the experimentally measured value of 15.3 kcal mol(-1). This present model allows for a clear distinction between contributions due to solvation and those due to intrinsic (gas-phase) effects and proves to yield results in very good agreement with available experimental data.

Ab initio study of the hydroxide ion–water clusters: An accurate determination of the thermodynamic properties for the processes nH2O+OH−→HO−(H2O)n (n=1–4)

Clusters of hydroxide ion, HO−(H2O)n=1–4, have been studied by high level ab initio calculations in order to better understand the first coordination shell of OH− ions. Geometry optimizations were performed at Hartree–Fock, density functional theory and second order Moller–Plesset perturbation theory levels using the 6-31+G(d,p) basis set. Single point energy calculations were carried out on the optimized geometries using the more extended 6-311+G(2df,2p) basis set and a higher level of electron correlation, namely fourth-order Moller–Plesset perturbation theory. For the n=1–3 clusters, only structures with the hydroxide ion hydrogen bonded to all waters molecules were considered. For the n=4 cluster, three minima were found; the most stable species has all four waters directly bound to the hydroxide ion, while the other two clusters have only three waters in the first coordination shell. In addition, the transition state connecting the cluster containing four waters in the first coordination shell to the...

Microwave Spectrum and Rotational Isomerism of Ethyl Formate

The existence of two rotational isomers of ethyl nitrate (CH3CH2ONO2) has been confirmed by gas phase microwave spectroscopy. The isomers are designated trans and gauche to indicate the relative positions of the CH3‐ and ‐NO2 groups. In the trans form all the heavy atoms lie in a plane. The gauche form differs from the trans by a rotation about the C–O bond of 95° and by an opening of the CON angle by ≈ 3°; both changes appear to indicate considerable nonbonded interaction of the CH3‐ and ‐NO2 groups. Stark effect measurements determined the dipole moments to be: μtrans = 3.39 D and μgauche = 3.23 D. The gauche form is less stable by 143 ± 70 cm−1. Several low‐lying excited states of the torsional vibration have been assigned for each form. A four‐term potential function for internal rotation about the C–O bond was determined. The uncertainties and error limits of a four‐term potential function are discussed.

The Gas-Phase Reaction Between Hydroxide Ion and Methyl Formate: A Theoretical Analysis of the Energy Surface and Product Distribution

The potential energy surface for the prototype solvent-free ester hydrolysis reaction: OH- +HCOOCH3 --> products has been characterized by high level ab initio calculations of MP4/6-311 + G(2df,2p)//MP2/6-31 + G(d) quality. These calculations reveal that the approach of an OH- ion leads to the formation of two distinct ion-molecule complexes: 1) the MS1 species with the hydroxide ion hydrogen bonded to the methyl group of the ester, and 2) the MS4 moiety resulting from proton abstraction of the formyl hydrogen by the hydroxide ion and formation of a three-body complex of water, methoxide ion and carbon monoxide. The first complex reacts to generate formate anion and methanol products through the well known B(AC)2 and S(N)2 mechanisms. RRKM calculations predict that these pathways will occur with a relative contribution of 85% and 15% at 298.15 K, in excellent agreement with experimentally measured values of 87% and 13%, respectively. The second complex reacts by loss of carbon monoxide to yield the water-methoxide complex through a single minimum potential surface and is the preferred pathway in the gas-phase. This water-methoxide adduct can further dissociate if the reactants have excess energy. These results provide clear evidence that the preferred pathways for ester hydrolysis in solution are dictated by solvation of the hydroxide ion.

Infrared multiple photon dissociation spectra of methanol-attached anions and proton-bound dimer cations

Abstract The infrared absorption spectra of several gas-phase ions solvated by a single methanol molecule have been determined indirectly by infrared multiple photon dissociation (IRMPD) spectroscopy. A modified White-type multi-pass cell was used in a Fourier transform ion cyclotron resonance mass spectrometer to record the disappearance of the parent ion as a function of the wavelength of a tunable CO2 laser operating in the range of 920–1060 cm−1. Spectra were obtained for fluoride, chloride and methoxide anions to which either a methanol or a d-methanol molecule was attached. In addition, spectra of proton-bound methanol and d-methanol dimer cations are presented. The shifts in the IRMPD peak frequencies are compared with the respective neutral infrared spectra. A consistent and interesting splitting is observed in the case of ions to which a methanol-d1 molecule is attached.

Selectivity and Mechanisms Driven by Reaction Dynamics: The Case of the Gas-Phase OH– + CH3ONO2 Reaction

Well-established statistical approaches such as transition-state theory based on high-level calculated potential energy profiles are unable to account for the selectivity observed in the gas-phase OH(-) + CH(3)ONO(2) reaction. This reaction can undergo bimolecular nucleophilic displacement at either the carbon center (S(N)2@C) or the nitrogen center (S(N)2@N) as well as a proton abstraction followed by dissociation (E(CO)2) pathway. Direct dynamics simulations yield an S(N)2:E(CO)2 product ratio in close agreement with experiment and show that the lack of reactivity at the nitrogen atom is due to the highly negative electrostatic potential generated by the oxygen atoms in the ONO(2) group that scatters the incoming OH(-). In addition to these dynamical effects, the nonstatistical behavior of these reactions is attributed to the absence of equilibrated reactant complexes and to the large number of recrossings, which might be present in several ion-molecule gas-phase reactions.