Interference of H-bonding and substituent effects in nitro- and hydroxy-substituted salicylaldehydes

Two intramolecular interactions, i.e., (1) hydrogen bond and (2) substituent effect, were analyzed and compared. For this purpose, the geometry of 4- and 5-X-substituted salicylaldehyde derivatives (X = NO₂, H or OH) was optimized by means of B3LYP/6-311 + G(d,p) and MP2/aug-cc-pVDZ methods. The results obtained allowed us to show that substituents (NO₂ or OH) in the para or meta position with respect to either OH or CHO in H-bonded systems interact more strongly than in the case of di-substituted species: 4- and 3-nitrophenol or 4- and 3-hydroxybenzaldehyde by ∼31%. The substituent effect due to the intramolecular charge transfer from the para-counter substituent (NO₂) to the proton-donating group (OH) is ∼35% greater than for the interaction of para-OH with the proton-accepting group (CHO). The total energy of H-bonding for salicylaldehyde, and its derivatives, is composed of two contributions: ∼80% from the energy of H-bond formation and ∼20% from the energy associated with reorganization of the electron structure of the systems in question.     

A B3LYP and MP2 theoretical investigation into host-guest interaction between calix[4]arene and Li+ or Na+

The DFT-B3LYP and MP2 methods with 6-311G** and 6-311++G** basis sets have been applied to study the complexation energies of the host-guest complexes between the cone calix[4]arene and Li⁺ or Na⁺ on the B3LYP optimized geometries. A comparison of the complexation energies obtained from the MP2(full) with those from MP2(fc) method is also carried out. The result shows that it is essential to introduce the diffuse basis set into the geometry optimizations and complexation energy calculations of the alkali-metal cation-π interaction complexes of calix[4]arene, and the D _e values show a maximum of 21.13 kJ mol⁻¹ (14.45% of relative error) between the MP2(full)/6-311++G** and MP2(fc)/6-311++G** method. For Li⁺ cation, the complexation is mainly energetically stabilized by the lower rim/cation (namely O–Li⁺) interaction. However, binding energies and NBO analyses confirm that Na⁺ cation prefers to enter the calix[4]arene cavity and the cation-π interaction is predominant, which contradicts the previous low-level theoretical studies. Furthermore, the complexation with Li⁺ is preferred over that with Na⁺ by at least 12.70 kJ mol⁻¹ at MP2(full)/6-311++G**//B3LYP/6-311++G** level.      

A MP2 and DFT study of the aromatic character of polyphosphaphospholes. Is the pyramidality the only factor to take into consideration?

A comprehensive MP2/6-311 + G(d,p) and B3LYP/6-311 + G(d,p) study of the aromatic character of phospholes, P _n (CH)_4-n PH with n = 0-4 was conducted. For this purpose, the structures for these compounds were optimized at both theoretical levels and different magnetic properties (magnetic susceptibility anisotropy, χ_anis, and the nucleus-independent chemical shifts, NICS) were evaluated. For comparison, these magnetic properties were also calculated in the optimized structures with planarity constraints. We have also applied the ACID (anisotropy of the current-induced density) method in this analysis. The main conclusions are the aromatic character of these compounds, the relationship between aromaticity and planarity and the importance of other factors in this aromaticity.     

Cooperative effect of water molecules in the self-catalyzed neutral hydrolysis of isocyanic acid: a comprehensive theoretical study

The detailed reaction mechanism for the water-assisted hydrolysis of isocyanic acid, HNCO + (n + 1) H₂O → CO₂ + NH₃ + nH₂O (n = 0−6), taking place in the gas phase, has been investigated. All structures were optimized and characterized at the MP2/6-31 + G* level of theory, and then re-optimized at MP2/6-311++G**. The seven explicit water molecules participating in the hydrolysis can be divided into two groups, one directly involved in the proton relay, and the other located in the vicinity of the substrate playing the cooperative role by engaging in hydrogen-bonding to HN = C = O. Two possible reaction pathways, the addition of water molecule across the C = N bond or across the C = O bond, are discussed, and the former is proved to be more favorable energetically. Our calculations suggest that, in the most kinetically favorable pathway for the titled hydrolysis, three water molecules are directly participating in the hydrogen transfer via an eight-membered cyclic transition state, while the other four water molecules catalyze the hydrolysis of HN = C = O by forming three eight-membered cooperative loops near the substrate. This strain-free hydrogen-bond network leads to the best estimated rate-determining activation energy of 24.9 kJ mol⁻¹ at 600 K, in excellent agreement with the gas-phase kinetic experimental result, 25.8 kJ mol⁻¹.     

Density functional theory characterisation of 4-hydroxyazobenzene

We report the structural properties, infrared (IR) and Raman spectra, dipole moment, polarisability, hardness and chemical potential of the trans and cis configurations of 4-hydroxyazobenzene calculated using the B3LYP functionals. All calculations were performed with the following basis sets: 6–31G, 6–31++G, 6–31G(d,p), 6–31++G(d,p), 6–31G(2d,2p), 6–31++G(2d,2p) and 6–311++G(2d,2p). We observed that 6–31++G(d,p) gives similar results to 6–311++G(2d,2p). Consequently, SVWN and PW91 methods were also used in association with 6–31++G(d,p) to test the influence of the different models of exchange and correlation functionals. A planar structure was obtained for all the optimised trans configuration structures. In both isomers, the presence of the hydroxyl group leads to an asymmetry in certain structural parameters. From these results, two IR or Raman active frequencies can be used to easily distinguish trans and cis configurations. The trans configuration was found to be more stable than the cis configuration by 67 ± 2 kJ mol⁻¹ at 0 K. The difference of the dipole moment between trans and cis for 4-hydroxyazobenzene was found to be lower than for trans and cis azobenzene.     

Ab initio studies on the reactivity of the CF3OCH2O radical: Thermal decomposition vs. reaction with O2

Hydrofluoroethers are being considered as potential candidates for third generation refrigerants. The present investigation involves the ab initio quantum mechanical study of the decomposition mechanism of CF₃OCH₂O radical formed from a hydrofluoroether, CF₃OCH₃ (HFE-143a) in the atmosphere. The geometries of the reactant, products and transition states involved in the decomposition pathways are optimized and characterized at the DFT (B3LYP) level of theory using 6-311G(d,p) basis set. Energy calculations have been performed at the G2(MP2) and G2M(CC,MP2) level of theory. Two prominent decomposition channels, C-O bond scission and reaction with atmospheric O₂ have been considered for detailed investigation. Studies performed at the G2(MP2) level reveals that the decomposition channel involving C-O bond scission occurs with a barrier height of 23.8 kcal mol⁻¹ whereas the oxidative pathway occurring with O₂ proceeds with an energy barrier of 7.2 kcal mol⁻¹. On the other hand the corresponding values at G2M(CC,MP2) are 24.5 and 5.9 kcal mol⁻¹ respectively. Using canonical transition state theory (CTST) rate constants for the two pathways considered are calculated at 298 K and 1 atm pressure and found to be 5.9 × 10⁻⁶ s⁻¹ and 2.3 × 10⁻⁵ s⁻¹ respectively. The present study concludes that reaction with O₂ is the dominant path for the consumption of CF₃OCH₂O in the atmosphere. Transition states are searched and characterized on the potential energy surfaces involved in both of the reaction channels. The existence of transition state on the corresponding potential energy surface is ascertained by performing intrinsic reaction coordinate (IRC) calculation.     

A MP2 and DFT study of the influence of complexation on the aromatic character of phosphole

This work is focused in three topical subjects: intermolecular interactions, metal ions, and aromaticity. A comprehensive MP2/6-31 + G* and B3LYP/6-31 + G* study of the influence of cation-π interactions on the aromatic character of phosphole was conducted. For this purpose, the structures of complexes were optimized at both theoretical level and different magnetic properties were evaluated. The main conclusion is the increase of the aromatic character of the phosphole when complexes with Li⁺, Be²⁺, and Al³⁺ are formed.     

Ab initio and DFT study on the electrophilic addition reaction of bromine to tetracyclo[5.3.0.02,6.03,10]deca–4,8–diene

The electronic and geometric structures of tetracyclo[5.3.0.0^2,6.0^3,10]deca-4,8-diene (hypostrophene) have been investigated by ab initio and DFT/B3LYP methods using the 6-31G* and 6-311G* basis sets. The double bonds of hypostrophene are endo-pyramidalized. The cationic intermediates and products formed in the addition reaction have been investigated using the HF/6-311G*, HF/6-311G**, and B3LYP/6-311G* methods. The bridged bromonium cation was more stable than the U-type cation. Considering that the bridged cation does not isomerize to the less stable U-type cation, it is not possible for the U-type product to be obtained in the reaction. The bridged bromonium cation transformed into the more stable N-type cation and the N-type product was obtained via this cation. The thermodynamic stability of the exo, exo and exo, endo isomers of the N-type dibromide molecule were almost identical. The N-type product was 16.6 kcal mol⁻¹ more stable than the U-type product. Figure General energy diagram of the hypostrophene–bromine (HS–Br₂) system (kcal mol⁻¹) (MP2/6-311G*//HF/6-311G*)     

An assessment to evaluate the validity of different methods for the description of some corrosion inhibitors

New research and development efforts using computational chemistry in studying an assessment of the validity of different quantum chemical methods to describe the molecular and electronic structures of some corrosion inhibitors were introduced. The standard and the highly accurate CCSD method with 6-311++G(d,p), ab initio calculations using the HF/6-31G++(d,p) and MP2 with 6-311G(d,p), 6-31++G(d,p), and 6-311++G(2df,p) methods as well as DFT method at the B3LYP, BP86, B3LYP*, M06L, and M062x/6-31G++(d,p) basis set level were performed on some triazole derivatives and sulfur containing compounds used as corrosion inhibitors. Quantum chemical parameters, such as the energy of the highest occupied molecular orbital energy (E_HOMO), the energy of the lowest unoccupied molecular orbital energy (E_LUMO), energy gap (ΔE), dipole moment (μ), sum of total negative charges (TNC), chemical potential (Pi), electronegativity (χ), hardness (η), softness (σ), local softness (s), Fukui functions (f ⁺,f ^?), electrophilicity (ω), the total energy change (?E_T) and the solvation energy (S.E), were calculated. Furthermore, the accuracy and the applicability of these methods were estimated relative to the highest accuracy and standard CCSD with 6-311++G(d,p) method. Good correlations between the quantum chemical parameters and the corresponding inhibition efficiency (IE%) were found.     

Theoretical evaluation of the order of reactivity of transfer agents utilized in RAFT polymerization

Density functional theory, DFT, PBE1PBE functional and 6−31+G(d,p) basis set in Gaussian 03 software were used in order to determine the reactivity order of the R group in RAFT agents used in the radical addition-fragmentation tranfer polymerization, through the evaluation of reactivity parameters such as: global and local electronegativity, hardness, softness, and philicity. It was found that the reactivity order is governed by both the number and the composition of the substituent group (primary, secondary or tertiary); that is, the larger those parameters are the larger factors like steric hindrance, polar effects and electronic interchanges are, which favors the breaking of the C−S bond from the adduct radical, permitting the exit of the leaving radical and allowing, as a consequence, the fragmentation step in RAFT polymerization. Trisubstituted dithioesters with structure S = C(Z)S−R, where Z = Phenyl and R = C(CH₃)₂CONH₂, C(CH3)2Ph or (CH₃)₂C₆H₈OCH₃, in accordance with the previously exposed, presented the most favorable reactivity parameters.     

An improved generalized AMBER force field (GAFF) for urea

We describe an improved force field parameter set for the generalized AMBER force field (GAFF) for urea. Quantum chemical computations were used to obtain geometrical and energetic parameters of urea dimers and larger oligomers using AM1 semiempirical MO theory, density functional theory at the B3LYP/6-31G(d,p) level, MP2 and CCSD ab initio calculations with the 6-311++G(d,p), aug-cc-pVDZ, aug-cc-pVTZ, and aug-cc-pVQZ basis sets, and with the CBS-QB3 and CBS-APNO complete basis set methods. Seven different urea dimer structures were optimized at the MP2/aug-cc-pVDZ level to obtain accurate interaction energies. Atomic partial charges were calculated at the MP2/aug-cc-pVDZ level with the restrained electrostatic potential (RESP) fitting approach. The interaction energies computed with these new RESP charges in the force field are consistent with those obtained from CCSD and MP2 calculations. The linear dimer structure calculated using the force field with modified geometrical parameters and the new RESP charge set agrees well with available experimental data.     

A systematical comparison of DFT methods in reproducing the interaction energies of halide series with protein moieties

A systematic theoretical investigation on the interaction energies of halogen-ionic bridges formed between halide ions and the polar H atoms bonded to N of protein moieties has been carried out by employing a variety of density functional methods. In this procedure, full geometry optimizations are performed at the Møller-Plesset second-order perturbation (MP2) level of theory in conjunction with the Dunning’s augmented correlation-consistent basis set, aug-cc-pVDZ. Subsequently, two distinct basis sets, i.e. 6-311++G(df,pd) and aug-cc-pVTZ, are employed in the following single-point calculations so as to check the stability of the results obtained at the different levels of DFT. The performance of DFT methods has been evaluated by comparing the results with those obtained from the rigorous MP2 theory. It is shown that the B98, B97-1, and M05 give the lowest root-mean-square error (RMSE) for predicting fluoride-binding energies, M05-2X, MPW1B95, and MPW1PW91 have the best performance in reproducing chloride-binding energies, B97-1, PBEKCIS, and PBE1KCIS present the optimal result for bromide-binding energies, while B97-1, MPW1PW91, and TPSS perform most well on iodide-binding energies. The popular B3LYP functional seems to be quite modest for studying halide-protein moiety interactions. In addition, the PBE1KCIS functional provide accuracies close to the computationally expensive MP2 method for the calculation of interaction energies of all halide-binding systems.     

Ab initio studies on the decomposition kinetics of CF<Subscript>3</Subscript>OCF<Subscript>2</Subscript>O radical

The present study deals with the decomposition of CF₃OCF₂O radical formed from a hydrofluoroether, CF₃OCHF₂ (HFE-125), in the atmosphere. The study is performed using ab initio quantum mechanical methods. Two plausible pathways of decomposition of the titled species have been considered, one involving C-O bond scission and the other occurring via F atom elimination. The geometries of the reactant, products and transition states involved in the decomposition pathways are optimized and characterized at DFT (B3LYP) level of theory using 6-311G(d,p) basis set. Single point energy calculations have been performed at G2M(CC,MP2) level of theory. Out of the two prominent decomposition channels considered, the C-O bond scission is found to be dominant involving a barrier height of 15.3 kcal mol⁻¹ whereas the F-elimination path proceeds with a barrier of 26.1 kcal mol⁻¹. The thermal rate constants for the above two decomposition pathways are evaluated using canonical transition state theory (CTST) and these are found to be 1.78 × 10⁶ s⁻¹ and 2.83 × 10⁻⁷ s⁻¹ for C-O bond scission and F-elimination respectively at 298 K and 1 atm pressure. Transition states are searched on the potential energy surfaces involved during the decomposition channels and each of the transition states is characterized. The existence of transition states on the corresponding potential energy surface is ascertained by performing intrinsic reaction coordinate (IRC) calculation.     

Extrapolation of water and formaldehyde harmonic and anharmonic frequencies to the B3LYP/CBS limit using polarization consistent basis sets

The harmonic and anharmonic frequencies of fundamental vibrations in formaldehyde and water were successfully estimated using the B3LYP Kohn-Sham limit. The results obtained with polarization- and correlation-consistent basis sets were fitted with a two-parameter formula. Anharmonic corrections were obtained by a second order perturbation treatment (PT2). We compared the performance of the PT2 scheme on the two title molecules using SCF, MP2 and DFT (BLYP, B3LYP, PBE and B3PW91 functionals) methods combined with polarization consistent pc-n (n = 0, 1, 2, 3, 4) basis sets, Dunning’s basis sets (aug)-cc-pVXZ where X = D, T, Q, 5, 6 and Pople’s basis sets up to 6-311++G(3df,2pd). The influence of SCF convergence level and density grid size on the root mean square of harmonic and anharmonic frequency deviations from experimental values was tested. The wavenumber of formaldehyde CH₂ anharmonic asymmetric stretching mode is very sensitive to grid size for large basis sets; this effect is not observed for harmonic modes. BLYP-calculated anharmonic frequencies consistently underestimate observed wavenumbers. On the basis of formaldehyde anharmonic frequencies, we show that increasing the Pople basis set size does not always lead to improved agreement between anharmonic frequencies and experimental values.     

The Hydronium Tetrafluoroborate Dimer in Nonpolar Media and its Proton NMR Spectrum

Hydronium tetrafluoroborate ion pairs, H₃O⁺·BF₄ ^- have been shown computationally to be unstable toward decomposition, in the absence of solvation or electrostatic interactions existing in crystals. As the proton NMR spectrum of a hydronium salt with the octanesulfonate-antimony pentachloride complex anion was reported in freon solution, we investigated the hypothesis that larger ionic clusters were present in the nonpolar solvent. It was found that the dimer (H₃O⁺·BF₄ ^-)₂ was stable at the MP2/6-31G* level. GIAO-B3LYP chemical shift calculations with the same basis set and also with the 6-31G**, 6-31++G**, 6-311++G**, dzvp, tzp, tz2p, and qz2p basis sets conducted on the hydronium fluoroborate dimer reproduce the main features of the experimental spectrum: the existence of two signals with a two-to-one intensity ratio and the more intense resonance at higher frequency (more deshielded). The alternative structures, of hydronium tetrafluoroborate ion pairs with one and with two hydrogen bonds between anion and cation, give calculated chemical shifts which are farther from the experimental values.     

The reaction of the aminoboranylidene-iminoborane isomerization: a CASSCF direct dynamics study

The barrier and the potential-energy surface of the isomerization from aminoboranylidene (BNH₂) to iminoborane (HBNH) have been studied using complete active space self-consistent field (CASSCF) with the 6−31+G(d, p) basis set and higher-level energy methods. The rate constants of the isomerization reaction are reported by employing the direct ab initio dynamics method. The geometries of all the stationary points were optimized using the B3LYP and CCSD methods with the cc-pVTZ and cc-pVQZ basis sets. The information along the intrinsic reaction coordinate (IRC) was also calculated at the CASSCF/6−31+G (d,p) level of theory. The energies were refined at the G3, G3MP2, G3MP2B3, CBS-Q, CBS-QB3, and two high-level (HL) methods based on the geometries optimized using CASSCF/6-31+G(d,p). The rate constants were evaluated using conventional transition-state theory (TST), canonical variational transition-state theory (CVT), and canonical variational transition-state theory with small curvature tunneling correction (CVT/SCT) and conventional transition-state theory with Eckart tunneling correction (TST/Eckart). According to the calculated results, we conclude that the tunneling effect is very important to this isomerization reaction.     

<Emphasis Type="Italic">Ab initio</Emphasis> Hartree-Fock calculations on linear and second-order nonlinear optical properties of new acridine-benzothiazolylamine chromophores

The calculation of optimized molecular structure and molecular hyperpolarizability of four new acridine-benzothiazolylamine chromophores (1–4) [2-nitro-6-(piperid-1-yl) acridine (1), 6-(benzothiazol-2-yl-amino)-2-nitro-acridine (2), 6-(6-ethylcarboxylate-benzothiazol-2-yl-amino)-2-nitroacridine (3), 6-(6-(β-hydroxyethyl-benzothiazol-2-yl-amino)-2-nitroacridine (4)] have been investigated using ab initio methods. Ab initio optimization were performed at the Hartree–Fock level using STO-3G basis set. The first hyperpolarizabilities have been calculated at the Hartree–Fock method with 6–31G and 6–311G basis sets using Gaussian 98W. In general, the first hyperpolarizability is dependent on the choice of method and basis set. To understand this phenomenon in the context of molecular orbital picture, we examined the frontier molecular orbital energies of all the molecules by using HF/6–31G, 6–311G levels. The polarizability, anisotropy of polarizability and ground state dipole moment of all the molecules have also been calculated. These acridine-benzothiazolylamine chromophores display significant second–order molecular nonlinearity, β (60.2–137.0 × 10⁻³⁰ esu) and provide the basis for future design of efficient nonlinear optical materials having the acridine-benzothiazolylamine core.     

MP2, density functional theory, and molecular mechanical calculations of C-H···π and hydrogen bond interactions in a cellulose-binding module-cellulose model system

Exploring non-covalent interactions, such as C-H···π stacking and classical hydrogen bonding (H-bonding), between carbohydrates and carbohydrate-binding modules (CBMs) is an important task in glycobiology. The present study focuses on intermolecular interactions, such as C-H?π (sugar-aromatic stacking) and H-bonds, between methyl β-d-glucopyranoside and l-tyrosine—a proxy model system for a cellulose-CBM complex. This work has made use of various types of quantum mechanics (QM) and molecular mechanics (MM) methods to determine which is the most accurate and computationally efficient. The calculated interaction potential energies ranged between −24 and −38 kJ/mol. The larger interaction energy is due to H-bonding between the phenyl hydroxyl of tyrosine and the O4 of the sugar. Density functional theory (DFT) methods, such as BHandHLYP and B3LYP, exaggerate the H-bond. Although one of the MM methods (viz. MM+) considered in this study does maintain the C-H?π stacking configuration, it underestimates the interaction energy due to the loss of the H-bond. When the O-H bond vector is in the vicinity of O4 (O-H?O4 ≈ 2 Å, e.g., in the case of MP2/6-31G(d)), the torsional energy drops to a minimum. For this configuration, natural bond orbital (NBO) analysis also supports the presence of this H-bond which arises due to orbital interaction between one lone pair of the sugar O4 and the σ∗(O-H) orbital of the phenyl group of tyrosine. The stabilization energy due to orbital delocalization of the H-bonded system is ∼13 kJ/mol. This H-bond interaction plays an important role in controlling the CH/π interaction geometry. Therefore, the C-H?π dispersive interaction is the secondary force, which supports the stabilization of the complex. The meta-hybrid DFT method, M05-2X, with the 6-311++G(d,p) basis set agrees well with the MP2 results and is less computationally expensive. However, the M05-2X method is strongly basis set dependent in describing this CH/π interaction. Computed IR spectra with the MP2/6-31G(d) method show blue shifts for C1-H, C3-H, and C5-H stretching frequencies due to the C-H?π interaction. However, the M05-2X/6-311++G(d,p) method shows a small red shift for the C1-H stretching region and blue shifts for the C2-H and C3-H stretches. For the aromatic tyrosine C_δ1-C_ε1 and C_δ2-C_ε2 bonds in the complex, the calculated IR spectra show red shifts of 12 cm⁻¹ (MP2/6-31G(d)) and 5 cm⁻¹ (M05-2X/6-311++G(d,p)). This study also reports the upfield shifts of computed ¹H NMR chemical shifts due to the C-H?π interaction.     

Structure and electronic properties of azadirachtin

We performed a combined DFT and Monte Carlo ¹³C NMR chemical-shift study of azadirachtin A, a triterpenoid that acts as a natural insect antifeedant. A conformational search using a Monte Carlo technique based on the RM1 semiempirical method was carried out in order to establish its preferred structure. The B3LYP/6-311++G(d,p), wB97XD/6-311++G(d,p), M06/6-311++G(d,p), M06-2X/6-311++G(d,p), and CAM-B3LYP/6-311++G(d,p) levels of theory were used to predict NMR chemical shifts. A Monte Carlo population-weighted average spectrum was produced based on the predicted Boltzmann contributions. In general, good agreement between experimental and theoretical data was obtained using both methods, and the ¹³C NMR chemical shifts were predicted highly accurately. The geometry was optimized at the semiempirical level and used to calculate the NMR chemical shifts at the DFT level, and these shifts showed only minor deviations from those obtained following structural optimization at the DFT level, and incurred a much lower computational cost. The theoretical ultraviolet spectrum showed a maximum absorption peak that was mainly contributed by the tiglate group.     

Halogen bonding and the design of new materials: organic bromides, chlorides and perhaps even fluorides as donors

In some halides RX, the halogen X has a region of positive electrostatic potential on its outermost portion, centered around the extension of the R−X bond. The electrostatic attraction between this positive region and a lone pair of a Lewis base is termed halogen bonding. The existence and magnitudes of such positive potentials on some covalently bonded halogens, and the characteristic directionality of the interaction, can be explained in terms of the degree of sp hybridization and polarizability of X and the electronegativity of R. Halogen bonding increases in strength in the order Cl < Br < I; fluorine is frequently said to not form halogen bonds, although a notable result of the present study is computational evidence that it does have the capability of doing so, if R is sufficiently electron withdrawing. An increasingly important application of halogen bonding is in the design of new materials (e.g., crystal engineering). In this paper, we present the calculated energies of a series of halogen-bonding interactions that could be the basis for forming linear chains, of types X----X----X---- or X----Y----X----Y----. We focus upon chlorides and bromides, and nitrogen bases. The B3PW91/6-311G(3df,2p) and MP2/6-311++G(3df,2p) procedures were used. We show how the computed electrostatic potentials (B3PW91/6-31G**) can provide guidance in selecting appropriate halide/base pairs. Figure Computed electrostatic potential of CH₃CH₂Br on the molecular surface defined by the 0.001-au contour of the electronic density. The bromine is facing the reader, and has a small positive (green) region centered around the intersection of the C–Br axis with the surface