Concerning the Lewis acidity of NO and NO2 as measured by their affinity to selected bases

Recently published theoretical results concerning the NO⁺ and NO₂⁺ gas-phase affinities of a few selected molecules, as well as the correlation between the above quantities and the corresponding proton affinities, are examined in the context of the much larger body of experimental and theoretical data already reported in the literature.     

On the way to understand antioxidants: chromanol and dimethoxyphenols gas-phase acidities

Some antioxidant mechanisms displayed by several phenolic compounds relate with OH bond dissociation energy. One way for its determination, in the gas-phase, relies on acidity measurements. Gas-phase acidities were determined experimentally, applying the kinetic method, for chromanol and four dimethoxyphenols, and theoretically through quantum chemical DFT calculations for chromanol, six dimethoxyphenols and 3,4,5-trimethoxyphenol. The experimental acidity order, 2,3-dimethoxyphenol > 3,5-dimethoxyphenol > 2,6-dimethoxyphenol > 3,4-dimethoxyphenol ≈ phenol > chromanol shows good agreement with the theoretical acidity order, 2,5-dimethoxyphenol > 2,3-dimethoxyphenol > 3,4,5-trimethoxyphenol > 3,5-dimethoxyphenol ≈ 2,4-dimethoxyphenol > 2,6-dimethoxyphenol > 3,4-dimethoxyphenol > phenol > chromanol. These acidity trends were rationalized in terms of the substituent effects on the thermodynamic stabilities both of the parent phenols and the corresponding phenoxide ions. DFT calculations also evidenced the occurrence of intramolecular C-H···O hydrogen bonds whenever there are vicinal substituents (either OH, O(-) or OCH(3)) which induce further stabilization of the geometries.     

Relative proton transfer abilities of acids and alcohols in gas phase and solutions

The present paper applies a method, based on SCF CNDO MO charge densities and assumption of the validity of the virial theorem, for calculation of relative proton affinities of alcohols, and substituted and unsubstituted carboxylic acids in the gas phase. The paper also chalks out a path for calculation of their relative acidities in solution phase by utilising a solvation energy equation and binding energy data in the gas phase. The results obtained by the present method are mostly in agreement with ion cyclotron resonance mass spectrometric experimental studies. The method has also been applied to cover the cases of amines in the gas phase.     

Quantum chemical studies on tautomerism of 2-, 3- or 4-hydroxyquinoline derivatives along with their thio and azo analogs

The geometries, relative stabilities and proton affinities for the different tautomers of 2-, 3- and 4-hydroxyquinoline derivatives and their thio and azo analogs along with their fixed forms (i.e. model molecules in which the proton migration is eliminated) were calculated with full geometry optimization using AM1, PM3 and MNDO methods. The predominance of oxo forms over hydroxy forms were confirmed with all three methods both in gas and liquid phases, as cited in the literature, with the exception of 3-hydroxyquinoline for which the AM1 and MNDO methods both in gas and liquid phases suggest the predominance of the hydroxy form. For the thio analogs the predominance of thione forms over mercapto forms and for the amino analogs the predominance of the amino forms over imino forms were confirmed both in gas and liquid phases with all three methods as indicated in the literature.     

Protonation of glycine and alanine: proton affinities, intrinsic basicities and proton transfer path

Proton affinities and intrinsic basicities for nitrogen and oxygen protonation in the gas phase of the amino acids glycine and alanine were calculated using density functional theory (DFT) and ab initio methods at different levels of theory from Hartree-Fock (HF) to G2 approximations. All methods gave good agreement for proton affinities for nitrogen protonation for both amino acids. However, dramatic differences were found between DFT, MP4//MP2, and G2 results on one hand, and MP4//HF results on the other to the calculation of structural and energetic characteristics of oxygen protonation in glycine and alanine. An investigation into the source of these differences revealed that electron correlation effects are chiefly responsible for the differences in calculated oxygen proton affinities between the various methods. It has been found that proton transfer between nitrogen and oxygen protonation sites in both amino acids occurs without a transfer path barrier when correlated methods were used to calculate the path energetics.     

A semi-empirical and ab initio combined approach for the full conformational searches of gaseous lysine and lysine–H2O complex

A full structural search of the canonical, zwitterionic, protonated and deprotonated lysine conformers in gas phase is presented. A total of 17,496 canonical, 972 zwitterionic, 11,664 protonated and 1458 trial deprotonated structures were generated by allowing for all combinations of internal single-bond rotamers. All the trial structures were initially optimized at the AM1 level, and the resulting structures were determined at the B3LYP/6-311G^* level. A total of 927 canonical, 730 protonated and 193 deprotonated conformers were found, but there were no stable zwitterionic structures in the gas phase. The most stable conformers of the canonical, protonated and deprotonated lysine were further optimized at the B3LYP/6-311++G^** level. The energies of the most stable structures were determined at the MP2/6-311G(2df,p) level and the vibrational frequencies were calculated at the B3LYP/6-311++G^** level. The rotational constants, dipole moments, zero-point vibrational energies, harmonic frequencies, vertical ionization energies, enthalpies, Gibbs free energies and conformational distributions of gaseous lysine were presented. Numerous new structures are found and the lowest-energy lysine conformer is more stable than the existing one by 1.1 kcal/mol. Hydrogen bonds are classified and may cause significant red-shifts to the associated vibrational frequencies. The calculated proton affinity/dissociation energy and gas-phase basicity/acidity are in good agreement with the experiments. Calculations are also presented for the canonical lysine–H₂O and zwitterionic lysine–H₂O clusters. Interaction between lysine and H₂O significantly affects the relative conformational stabilities. Only one water molecule is sufficient to produce the stable zwitterionic structures in gas phase. The lowest-energy structure is found to be zwitterions when applying the conductor-like polarized continuum solvent model (CPCM) to the lysine–H₂O complexes.     

Ratio of the equilibrium and kinetic acidities of solid catalysts

The feasibility of using the equation log k = const – αH _0s was examined for solid acid catalysts. Data for the thermoprogrammed dealkylation of cumene showed that the temperature dependence of the strength of the acid sites of the catalyst H _0s(T) should be considered in calculating the coefficient α. In this case, α→1. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 45, No. 3, pp. 173-175, May-June, 2009.     

Halogens in γ-position enhance the acidity of alkyl aryl sulfones and alkane nitriles

On the basis of measuring the rates of base-catalyzed deuterium exchange the pK_a values of a series of 3-halopropyl aryl sulfones and 4-halobutyronitriles were estimated. It was shown that halogen substituents, although separated from the carbanionic site, exert a substantial carbanion stabilizing effect. These effects were rationalized by DFT calculations.     

Strong dissimilarities between the gas-phase acidities of saturated and alpha,beta-unsaturated boranes and the corresponding alanes and gallanes

The effect that unsaturation has on the intrinsic acidity of boranes, alanes, and gallanes, was analyzed by B3 LYP and CCSD(T)/6-311+G(3df,2p) calculations on methyl-, ethyl-, vinyl-, and ethynylboranes, -alanes and -gallanes, and on the corresponding hydrides XH3. Quite unexpectedly, methylborane, which behaves as a carbon acid, is predicted to have an intrinsic acidity almost 200 kJ mol(-1) stronger than BH3, reflecting the large reinforcement of the C--B bond, which upon deprotonation becomes a double bond through the donation of the lone pair created on the carbon atom into the empty p orbital of the boron. Also unexpectedly, and for the same reason, the saturated and alpha,beta-unsaturated boranes are much stronger acids than the corresponding hydrocarbons, in spite of being carbon acids as well. The Al derivatives also behave as carbon acids, but in this case the most favorable deprotonation process occurs at C beta, leading to the formation of rather stable three-membered rings, again through the donation of the C beta lone pair into the empty p orbital of Al. For Ga-containing compounds the deprotonation of the GaH2 group is the most favorable process. Therefore only Ga derivatives behave similarly to the analogues of Groups 14, 15, and 16 of the periodic table, and the saturated derivatives exhibit a weaker acidity than the unsaturated ones. Within Group 13, boranes are stronger acids than alanes and gallanes. For ethyl and vinyl derivatives, alanes are stronger acids than gallanes. We have shown, for the first time, that acidity enhancement for primary heterocompounds is not only dictated by the position of the heteroatom in the periodic table and the nature of the substituent, but also by the bonding rearrangements triggered by the deprotonation of the neutral acid.     

3,4-二硒方酸芳香性和气相酸性的理论研究

在ＲＨＦ／６－３１１Ｇ水平优化得到３,４－二硒方酸（３,４－二氢硒基－３－环丁烯－１,２－二酮）３ 种平面构象异构体的平衡几何构型。进一步用ＭＰ２（ｆｕｌｌ）／６－３１１Ｇ／／ＲＨＦ／／６－３１１Ｇ方法计算单点能量,发现ＺＺ型异构体是能量最低构象,且ＺＺ和ＺＥ型能量非常接近。用优化的最稳定构象ＺＺ型异构体在ＲＨＦ／６－３１１Ｇ／／ＲＨＦ／６－３１１Ｇ、ＲＨＦ／６－３１１＋ Ｇ／／ＲＨＦ／６－３１１＋ Ｇ、ＭＰ２（ｆｕｌｌ）／６－３１１＋ Ｇ／／ＲＨＦ／６－３１１＋ Ｇ 和Ｂ３ＬＹＰ／６－３１１＋ Ｇ／／Ｂ３ＬＹＰ／６－３１１＋ Ｇ水平计算其气相酸性（ΔＧ°）和同键反应芳香性稳定化能（ＨＡＳＥ）。用基团加和法（ｇｒｏｕｐ ｉｎｃｒｅｍ ｅｎｔ ａｐ－ｐｒｏａｃｈ ） 在 ＲＨＦ／６－３１１ ＋ Ｇ／／ＲＨＦ／６－３１１ ＋ Ｇ 和 Ｂ３ＬＹＰ／６－３１１ ＋ Ｇ／／Ｂ３ＬＹＰ／６－３１１＋ Ｇ水平计算其磁化率增量（Λ）。计算结果指出标题化合物的同键反应芳香性稳定化能和磁化率增量均为负值,表明它具有芳香性,实现了标题化合物芳香性的几何、能量和磁性的判定。     

Catalysis by hetropoly compounds. Part XXVI. Gas phase synthesis of methyl tert-butyl ether over heteropolyacids

Gas phase synthesis of methyl tert-butyl ether (MTBE) from methanol and isobutylene has been studied with several heteropolyacids at 303–383 K. It was found that a Dawson-type heteropolyacid, H₆P₂W₁₈O₆₂, was much active than Keggin-type heteropolyacids, H_nXW₁₂O₄₀ (X = P, Si, Ge, B, and Co), and other solid acids such as SO²⁻₄/ZrO₂, SiO---Al₂O₃ and H-ZSM-5 at 323 K. Since the acid strength of H₆P₂W₁₈O₆₂ was weaker than H₃PW₁₂O₄₀ and H₄SiW₁₂O₄₀, factors other than the acid strength are important for the catalytic activity. Pseudoliquid phase behavior was demonstrated for H₆P₂W₁₈O₆₂ and H₃PW₁₂O₄₀ by the measurements of the absorption of methanol during the reaction and by the unique pressure dependencies of the rate of synthesis. From the absorption data (the amount and rate), it is concluded that the high catalytic activity of H₆P₂W₁₈O₆₂ is brought about by a high-activity state of the pseudoliquid phase in which controlled amounts of molecule are absorbed and the absorption-desorption is rapid. On the other hand, the pseudoliquid phase of H₃PW₁₂O₄₀ is in a low-activity state absorbing excessive amounts of molecule.     

Mind the correct basis set: A case study for predicting gas phase acidities of small compounds using calculations from first principles

Some of the most popular computational methods have been utilized to determine a dependency of the acidity trend of the first‐row hydrides on a choice of basis set. For about three decades, methyl anion ( ) was known as the strongest base but after Tian et al. were able to produce the gas phase lithium monoxide anion (LiO–) they discovered it was a stronger base than (Tian et al., Proc Natl Acad Soc USA 2008, 105, 7647). Furthermore, the authors confirmed their experimental results using high‐level ab initio methods, namely W1 and W2C composite methods, as well as complete active space‐averaged quadratic coupled cluster and Brueckner Doubles with triple excitation contribution (BD(T)) within the aug‐cc‐pVQZ basis set. These methods are highly demanding in terms of the computational effort as well as a level of expertise needed from the user to correctly conduct such calculations. We have shown that the proper acidity trend, that is, , can be obtained with less expensive, ”black‐box” type methods if only the basis set is properly chosen. Our results prove that the diffuse augmented basis sets are absolutely necessary for appropriate predictions of acidities. Our calculations show that the correct order of is achieved by augmenting relatively small cc‐pVXZ (X = D,T) basis sets. A similar effect is observed for the family of Pople's basis sets. Our estimate for with CCSD(T)/aug‐cc‐pVTZ was 423.8 kcal/mol, which agrees very well with the experimental value 425.7 ± 6.1 kcal/mol. An important finding is that the proper acidity trend may be reversed if the basis sets are not correctly selected. © 2014 Wiley Periodicals, Inc.     

氯丙基聚硅氧烷的合成及用作气相色谱固定相的研究

合成了几种新型的含氯丙基的取代聚硅氧烷,其热稳定性可达300～325℃,能较好地分离非极性和中等极性化合物。当固定相的苯基含量高达50％时,用8％的DCP能使其有效交联。     

Proton affinities of molecules containing nitrogen and oxygen: Comparing density functional results to experiment

Summary Proton affinities were calculated using density functional theory for 11 small molecules whose primary protonation site is on nitrogen, and eight small molecules that protonate on oxygen. Calculations were performed using both the local spin density approximation and nonlocal gradient corrections to the exchange correlation functional. The results were not sensitive to whether the nonlocal gradient correction was implemented on the final local spin density optimized geometry or whether the correction was included in the self-consistent calculation of the energy at each optimization step. Although negligible basis set dependence was found using the analytic Gaussian basis sets, numerical basis sets required augmentation by a double set of polarization functions to achieve reasonable agreement with experiment. All calculations systematically underestimated oxygen proton affinities.     

Gas Phase Hydrogen Spillover and Oxygen Content

A specially constructed all-glass reaction system was used to reinvestigate hydrogen spillover through the gas phase with extremely purified hydrogen, using alkene hydrogenations in the gas phase as test reactions. The spillover reaction could be observed even with hydrogen - with less than 0.01 ppm O₂ - which additionally was passed over the same type of Pt/Al₂O₃ catalyst at the beginning of the line. In a second group of experiments the effect of oxygen traces was investigated, which were added to the hydrogen stream. Small oxygen contents (50 ppm) did not influence the reaction, higher contents (100 ppm) caused reduced conversion.     

Gas chromatography on a self-associating component of a binary phase. Retention model by formal analogy with conductance of electrolytes in dilute solution

Summary The dependence of specific retention by a binary stationary phase in GC can be expressed as the sum of the products of the specific retention of the pure components times their respective volume fractions. In this study, however, one component has a site, which is not only mainly responsible for the selectivity, but also participates in strong self-association. This requires introduction of a concentration-dependent factor (^x) in the corresponding term of the equation correlating Vg _mix ^x with x. In the GC resolution of N-trifluoroacetyl-amino acid isopropyl esters on a binary phase, N-lauroyl-L-amino t-butyl amide-squalane, data for the values of ^x were obtained.Adapting a previously developed LC retention model to the above GC data, an equation was derived for the dependence of ^x on the weight fraction (x) of the selector, namely ^x=1/x. This relationship permits calculation of retention volumes on the binary phase for a given x, as well as corresponding resolution coefficients of enantiomeric amino acid derivatives with generally excellent agreement with experiment. The chirality of the system is not relevant to application of the equation.     

Gas chromatography on a self-associating component of a binary phase. Retention model by formal analogy with conductance of electrolytes in dilute solution

Summary The dependence of specific retention by a binary stationary phase in GC can be expressed as the sum of the products of the specific retention of the pure components times their respective volume fractions. In this study, however, one component has a site, which is not only mainly responsible for the selectivity, but also participates in strong self-association. This requires introduction of a concentration-dependent factor (μ^x) in the corresponding term of the equation correlating Vg _mix ^x with x. In the GC resolution of N-trifluoroacetyl-amino acid isopropyl esters on a binary phase, N-lauroyl-L-valine t-butyl amide-squalane, data for the values of μ^x were obtained. Adapting a previously developed LC retention model to the above GC data, an equation was derived for the dependence of μ^x on the weight fraction (x) of the selector, namely μ^x=√1/x. This relationship permits calculation of retention volumes on the binary phase for a given x, as well as corresponding resolution coefficients of enantiomeric amino acid derivatives with generally excellent agreement with experiment. The chirality of the system is not relevant to application of the equation.     

气相色谱-质谱法分析硫芥子气及相关化合物

用气相色谱－质谱法对含硫芥子气类化合物进行了分析。探讨了长链芥子气的电子轰击电离的质谱规律，并对待测化合物的色谱保留数据进行了研究，在给定条件下，各种待测含硫芥子气在气相色谱柱中能有效的分离和鉴定。     

Investigations on mixed water-clusters: Gas phase titration with cyclopentanone and identification of some isomers by time-of-flight mass spectrometry

Mixed-clusters of water with cyclopentanone have been investigated using high resolution time-of-flight mass spectrometry. These clusters are synthesized in a gas-aggregation source at comparatively higher temperature. They contain water-cluster at the core and cyclopentanone molecules hydrogen bonded through ketone oxygen with the dangling OH available at the core. Thus these mixed-clusters may also be considered as the products of a titration in gas phase. The growth reaction reveals that all clusters are protonated. From the configuration of dimer and tetramer, it is suggested that the proton resides as an Eigen ion in the core. The protonated mixed-clusters containing six, seven and eight water molecules substantiate the hydronium contained hexa, hepta and octamer water-cluster structures predicted by [KJ(H₃O)⁺] model calculations. For clusters with 9–19 water molecules, the core appears to have configurations that give less than the predicted number of dangling bonds. In large size clusters having more than 20 water molecules, the water-core appears to have open configuration like the melted structures obtained as a result of increase in temperature.     

The structures of HCOO−, CH3COO−, C2H5COO− and CH3O− in gas phase and in crystal structure by ab initio and resonance theory

The purpose of this report is to quantitatively find the cause for the elongation of the R-C bond in R-COO^– (R = H, CH₃ and C₂H₅) and the shortening of the C-O bond in CH₃-O^– upon deprotonation in the gas phase. These elongations and shortenings result from the contributions of R^–---CO₂ and H^–---CH₂=O as resonance structures to the systems. Because these structures must make only a small contribution in the crystal, the R-C bond lengths of R-COO^– (R= H and CH₃) in the crystal structure are shorter than those in the gas phase.