Analysis of methods for calculating the transition frequencies of the torsional vibration of acrolein isomers in the ground (S 0) electronic state

B3LYP, MP2, CCSD(T), and MP4/MP2 in the 6-311G(d, p), 6-311++G(d, p), cc-pVTZ, aug-cc-pVTZ bases used to calculate the transition frequencies of torsional vibration of trans- and cis-isomers of acrolein in the ground electronic state (S ₀) are analyzed. It is found that for trans-isomers, all methods of calculation except for B3LYP in the cc-pVTZ basis yield good agreement between the calculated and experimental values. It is noted that for the cis-isomer of acrolein, no method of calculation confirms the experimental value of the frequency of torsional vibration (138 cm^?1). It is shown that the calculated and experimental values for obertones at 273.0 cm^?1 and other transitions of torsional vibration are different for this isomer in particular. However, it is established that in some calculation methods (B3LYP, MP2), the frequency of the torsional vibration of the cis-isomer coincides with another experimental value of this frequency (166.5 cm^?1). It is concluded that in analyzing the vibrational structure of the UV spectrum, the calculated and experimental values of its obertone (331.3 cm^?1) coincide, along with its frequency. It is also noted that the frequency of torsional vibration for the cis-isomer (166.5 cm^?1) can also be found in other experimental works if we change the allocation of torsional transition 18 ₁ ¹ .     

Quantum-chemical studies of methyl and silyl torsional frequencies and the structures of disilylsulphide and trisilylphosphine

Torsional frequencies of methyl and silyl groups occurring in a range of molecules have been calculated by HF, B3LYP and MP2 methods using several basis sets. Linear correlations between calculated and observed values are derived and used to predict unobserved or dubious frequencies. The current experimental value for the E torsion in trimethylphosphine is questioned. The relative merits of the B3LYP and MP2 methods are explored. MP2 calculations can show wide variation with respect to basis set. In cases where two or more silyl groups are attached to a common atom, as in disilylsulphide (SiH3)2S, disilylmethane (SiH3)2CH2, trisilylmethane (SiH3)3CH and tetrasilylmethane (SiH3)4C, marked differences occur between B3LYP and MP2 estimates. These may be linked to concomitant differences in conformation or potential barrier restraining internal rotation. In disilylmethane the B3LYP results agree much better with experiment than those from the MP2 method. HF and B3LYP calculations for disilylsulphide and trisilylphosphine give normal C2v and C3v equilibrium structures, respectively, but in MP2 structures the silyl groups are twisted through 6-13 degrees yielding C2 and C3 configurations. It may be possible to distinguish between these structures through the observation of isolated SiH stretching frequencies in the spectra of fully deuterated materials. Dispersion forces could contribute to the twisting calculated by the MP2 method. Further studies of the microwave and vibrational spectra of disilylsulphide and trisilylphosphine isotopomers are needed.     

Redox potentials of dopamine and its supramolecular complex with aspartic acid

Dopamine (DA) can be oxidized to dopamine quinone (DAquinone) through a one-step, two-electron redox reaction. The electron transfer property of DA and its supramolecular complex with aspartic acid (Asp) has been investigated by the theoretical calculations. We calculated the standard redox potentials (E ^o) of DA/DAquinone at the MP2/6-31G(d,p)//B3LYP/6-31G(d,p), MP2/6-31+G(d,p)//B3LYP/6-31+G(d,p), MP2/6-31G(d,p)//B3LYP/6-311G(d,p), and MP2/6-311+G(d,p)//B3LYP/6-311+G(d,p) levels. Comparing the experimental value, the redox potentials of DA/DAquinone obtained at MP2//B3LYP/6-311G(d,p) and MP2//B3LYP/6-311+G(d,p) levels can be considered as the upper and lower estimates. DA can form supramolecular complex (DA-Asp) with Asp through hydrogen bond (H-bond). Therefore, the values of 0.631 and 0.628 V obtained at MP2//B3LYP/6-311G(d,p) and MP2//B3LYP/6-311+G(d,p) levels for DA-Asp/DAquinone-Asp can be proposed as the upper and lower estimates of a probable (about 0.630 V) value of the corresponding redox potential. The calculated E ^o values of DA-Asp/DAquinone-Asp at the four theoretical levels are upper than those of DA/DAquinone, which indicates that the formation of H-bonds weaken the electron-donating ability of DA.     

Ab initio- and density-functional studies of conformational behaviour of N-formylmethionine in gaseous phase

The current work is a study of the conformational space of the non-ionic N-formylmethionine molecule around its seven structurally significant internal backbone torsional angles at B3LYP/6-31++G(d,p) levels of theory in the gaseous phase. The potential energy surface exploration reveals that a total of 432 different conformers would result if all the possible combinations of the internal rotations were to be considered. A set of twelve conformers of the N-formylmethionine molecule are then further analysed in terms of their relative stabilities, theoretically predicted harmonic vibrational frequencies, HOMO-LUMO energy gaps, ESP charges, rotational constants and dipole moments calculated using MP2/6-31++G(d,p) and B3LYP/6-311++G(d,p) levels. The calculated relative energy-range of the conformers at the MP2 level is 11.08 kcal mol^?1 (1 kcal = 4.1868 kJ), whereas the same obtained at the B3LYP level is 10.02 kcal mol^?1. The results of this study provide a good account of the role of four types of intramolecular H-bonds, namely O…H—O, O…H—N, O…H—C and N…H—C, in influencing the energies of the conformers as well as their conformational and vibrational spectroscopic aspects. The relative stability order of the conformers appears to depend on the level of theory used while the vibrational frequencies calculated at the B3LYP level are in better agreement with the experimental values.     

Second-sphere coordination in non-spherical anion binding: Synthesis, characterization and X-ray structure of cis-diazidobis(ethylenediamine)cobalt(III) 2-chloro,5-nitrobenzenesulphonate monohydrate

The dipole, quadrupole, and other second moments have been determined at the equilibrium structures of the halogen azides, isocyanates, and isothiocyanates, for both principal axes and inertial axes. The theoretical procedures used are Möller–Plesset (MP2) and DFT (B3LYP) methodologies, with TZVP and cc-pVTZ basis sets. There is systematic variation in the calculated directions of the dipole moments in the present series, where B3LYP and MP2 methodologies show differences up to ∼20° for the directions. This discrepancy is largest in ClN₃, but quite significant for several other compounds, such as XNCO (where X = Cl, Br and I). The dipole moments of the compounds rotate through a wide angle, as the halogen changes; in contrast, the axes of the second moments rotate to a much smaller degree. The properties are compared with the limited microwave spectral data so far available, in the hope that the present study will encourage further experimental study. There is an urgent need for new experimental data on the dipole moment a,b-components for these compounds.     

Vibrational spectra and conformations of 1,4-cyclohexadiene and its oxygen analogues: ab initio and density functional calculations

The vibrational spectra and ring-puckering potential energy functions of 1,4-cyclohexadiene, 4H-pyran and 1,4-dioxin have been examined using a density functional theory (DFT) method as well as the Hartree–Fock (HF) and second-order Møller–Plesset (MP2) methods. The calculated vibrational frequencies and potential energy functions of those molecules have been compared with previously reported experimental data and MM3 results. For all three molecules, the DFT method using Becke's three-parameter functional (B3LYP) has led to the prediction of more accurate vibrational frequencies than the HF and MP2 methods. The enlargement of the basis set at the B3LYP levels has improved the accuracy of calculated vibrational frequencies. In particular, the C–O–C=C torsional force field parameters obtained from the B3LYP method have correctly predicted the ring-puckering potential energy functions of the oxygen-containing analogues, 4H-pyran and 1,4-dioxin, which could not be done by the MM3 method.     

Theoretical study in gas phase of linear and nonlinear optical properties of the ortho-, meta- and para-nitrophenol isomers

The linear (α), and nonlinear (β, γ) optical NLO properties of ortho-, meta- and para-nitrophenol (ONP, MNP and PNP) isomers have been calculated in gas phase by using ab initio (HF, MP2 and MP4) and density functional theory (DFT) (B3LYP, CAM-B3LYP) methods, with the 6-31+G(d,p) and 6-311+G(3d,3p) standard and the Sadlej specialized basis sets. These properties were evaluated both at static and at dynamic regime within the finite field FF numerical techniques and the time-dependent-Hartree–Fock approach at 1,910 nm, respectively. Additional calculations were performed for the β static hyperpolarizability of these isomers in presence of p-dioxane solvent with the Onsager Model and the SCRF-PCM approach, using B3LYP/6-31+G(d,p) and MP2/6-31+G(d,p) levels of theory. Additionally, CCSD/6-31+G(d,p) calculations were performed for the α, β and γ properties of PNP isomer. The B3LYP and MP2 α _ave results of the nitrophenol isomers are comparable to the experimental α _ave reports; while the tendency for the β _v calculated values (β _v PNP > β _v MNP > β _v ONP), that can be explained in terms of the O _x atomic charge of the –NO₂ group, does not follow exactly the experimental ones. The B3LYP γ _ave results are in correspondence to the experimental measurements, the correlation of which is r ² = 0.99. The use of FF methodology in conjunction with the B3LYP and MP2 methods and the 6-31+G(d,p) basis set show to be appropriate approaches to predict qualitative optical properties of Push–Pull like organic molecules, provided are considered the solvent effects or frequency dependence. However, to have a clear picture of the NLO properties of an isolated molecule, higher order correlation effects combined with specialized basis sets, frequency and solvent effects should be employed. We have demonstrated that MP4/Sadlej level of theory is able to reproduce NLO properties that can be considered equivalent to those from more sophisticated approaches, such as CCSD together with extended basis sets.     

A theoretical study of nonlinearity in heterocyclic hydrogen-bonded complexes

Ab initio calculations are performed at the MP2/6-311++G(d,p) and DFT/B3LYP/6-311++G(d,p) theoretical levels to obtain geometries, H-bond energies and harmonic infrared vibrational properties for the Cs symmetry structures of heterocyclic hydrogen-bonded complexes, CnHmY-HX. The H-bond lengths in DFT/B3LYP calculation level are in better agreement with the experimental values than the MP2 results. The geometry optimization are interpreted in terms of hydrogen bond nonlinearity represented by theta; and phi angles, once the hydrogen bond is formed among n-electrons pairs of the heteroatom in heterocyclic and the hydrogen atom in HX. The hydrogen bond energy after of the zero-point vibrational energy (ZPE) and basis set superposition error (BSSE) corrections are overestimated at DFT/B3LYP, whereas the MP2 BSSE corrections are very large than corresponding DFT/B3LYP. For example, the BSSE corrections for the C2H4S-HNC complex are 7.60 and 0.09 kJ mol(-1) in MP2 and DFT/B3LYP calculations levels, respectively. The new vibrational modes in infrared harmonic spectrum arising from complexation show several interesting features, especially the intermolecular stretching mode.     

Conformations,vibrational spectra and force field of 1-methyl-2-(2′-pyridyl)benzimidazole: experimental data and density functional theory investigation in comparison with 2-(2′-pyridyl)benzimidazole

The molecular structure, conformational equilibria, vibrational spectra and molecular force field of 1-methyl-2-(2′-pyridyl)benzimidazole have been determined at the HF, MP2 and DFT/(B3LYP, BVP86) levels with 6-31+G(d,p) and TZVP basis sets. The torsional potentials for the rotation around the C1–C2 pivotal bond have been calculated at the B3LYP/6-31+G(d,p) and BVP86/TZVP levels of theory for gaseous and aqueous 1-methyl-2-(2′-pyridyl)benzimidazole. FT-Raman (3500–10 cm^?1) and FT-IR (3900–400 cm^?1) spectra of solid 1-methyl-2-(2′-pyridyl)benzimidazole have been recorded and interpreted on a base of calculated potential energy distribution. The results of the experimental and theoretical study of vibrational spectra and molecular structure of 1-methyl 2-(2′-pyridyl)benzimidazole are considered in comparison with similar data for 2-(2′-pyridyl)benzimidazole.

     

Molecular structure,conformational stability,energetic and intramolecular hydrogen bonding in ground,and electronic excited state of 3-mercapto propeneselenal

In the present work, a conformational analysis of 3-mercapto propeneselenal is performed using several computational methods, including DFT (B3LYP), MP2, and G2MP2. At the DFT and G2MP2 levels the most stable conformers of title compound are characterized by an extended backbone structure, minimizing the steric repulsions between the sulfur and selenium lone pairs. Two conformers exhibit hydrogen bonding. This feature, although not being the dominant factor in energetic terms, appears to be of foremost importance to define the geometry of the molecule. The influence of the solvent on the stability order of conformers and the strength of intramolecular hydrogen bonding was considered using the PCM, SCI–PCM, and IEF–PCM methods. The results of analysis by quantum theory of “Atoms in Molecules” and natural bond orbital method fairly support the DFT results. The calculated HOMO and LUMO energies showed that charge transfer occurs within the molecule. Further verification of the obtained transition state structures was implemented via intrinsic reaction coordinate analysis. Calculations of the ¹H NMR chemical shift at GIAO/B3LYP/6–311++G** levels of theory are also presented. The excited-state properties of intramolecular hydrogen bonding in hydrogen-bonded systems have been investigated theoretically using the time-dependent density functional theory method.     

Conformational study of the structure of 12-crown-4-alkali metal cation complexes

A conformational search was performed for the 12-crown-4 (12c4)-alkali metal cation complexes using two different methods, one of them is the CONFLEX method, whereby eight conformations were predicted. Computations were performed for the eight predicted conformations at the HF/6-31+G*, MP2/6-31+G*//HF/6-31+G*, B3LYP/6-31+G*, MP2/6-31+G*//B3LYP/6-31+G*, and MP2/6-31+G* levels. The calculated energies predict a C4 conformation for the 12c4-Na+, -K+, -Rb+, and -Cs+ complexes and a C(s) conformation for the 12c4-Li+ complex to be the lowest energy conformations. For most of the conformations considered, the relative energies, with respect to the C4 conformation, at the MP2/6-31+G*//B3LYP/6-31+G* are overestimated, compared to those at the MP2/6-31+G* level, the highest level of theory considerd in this report, by 0.2 kcal/mol. Larger relative energy differences are attributed to larger differences between the B3LYP and MP2 optimized geomtries. Binding enthalpies (BEs) were calculated at the above-mentioned levels for the eight conformations. The agreement between the calculated and experimental BEs is discussed.     

Reaction energetics of tetrahedrane and other hydrocarbons: Ab initio and density functional treatments

Reaction energetics of the highly strained tetrahedrane molecule, as well as some smaller hydrocarbons, were computed at the ab initio MP4, QCISD (T), G1, and G2 levels and also by several density functional (DF) approaches: VWN(LDA), PW86, BLYP, and B3LYP. A variety of basis sets were used in the DF calculations. For atomization processes, nonlocal DF procedures are superior to MP4 and QCI. For the other reactions, B3LYP is the most reliable DF approach and is overall competitive with MP4 and QCISD (T), although the MP4/6-311G(2df, p) results are generally the closest to the experimental and/or the G2 values. There is no consistent correlation between the accuracy of the DF results and the size of the basis set. © 1996 John Wiley & Sons, Inc.     

Theoretical study of the vibrational frequencies of carbon disulfide

A benchmark comparison for different computational methods and basis sets has been presented. In this study, five computational methods (Hartree–Fock (HF), MP2, B3LYP, MPW1MP91, and PBE1PBE) along with 18 basis sets have been applied to optimize the geometry of carbon disulfide (CS₂), and further calculate the vibrational frequencies of the optimized geometries. The differences between the calculated frequencies and corresponding experimental data are used to evaluate the efficiency of each combination of computational method and basis set. The comparison of frequency difference indicates that B3LYP generally gives the best prediction of frequencies for CS₂, whereas the other two density functional theory (DFT) methods, i.e., MPW1PW91 and PBE1PBE, often give parallel results. Although MP2 predicts the frequencies with accuracy almost as good as those from DFT methods, in a particular case, HF calculation outperforms MP2 as well as MPW1PW91 and PBE1PBE for prediction of the frequency of asymmetrical stretching for CS₂. © 2013 Wiley Periodicals, Inc.     

Thermodynamic functions of lactams in the ideal gas state

Thermodynamic functions (enthalpy, entropy, free energy, and heat capacity) of azacycloalkan-2-ones with ring sizes n = 4–8 in the ideal gas state are calculated by means of quantum chemistry and statistical physics, using an anharmonic approximation in the range of 298–1500 K with allowance for all known conformers and optical isomers. Equilibrium structures and total energies of lactams are calculated using the B3LYP/6-311++G(3df, 3pd), B3LYP/aug-cc-pVQZ, and MP2/6-311++G(3df, 3pd) methods, and the anharmonic frequencies of the fundamental vibrations of all the investigated structures were found via B3LYP/6-311++G(3df, 3pd).     

Conformational analysis and rotational barriers of alkyl- and phenyl-substituted urea derivatives

Potential energy surfaces (PES) for rotation about the N-C(sp(3)) or N-C(aryl) bond and energies of stationary points on PES for rotation about the C(sp(2))-N bond are reported for methylurea, ethylurea, isopropylurea, tert-butylurea, and phenylurea, using the B3LYP/DZVP2 and MP2/aug-cc-pVDZ methods. The analysis of alkylureas reveals cis and (less stable) trans isomers that adopt anti geometries, whereas syn geometries do not correspond to stationary points. In contrast, the analysis of phenylurea reveals that the lowest energy form at the MP2 level is a trans isomer in a syn geometry. The fully optimized geometries are in good agreement with crystal structure data, and PESs are consistent with the experimental dihedral angle distribution. Rotation about the C(sp(2))-N bond in alkylureas and phenylurea is slightly more hindered (8.6-9.4 kcal/mol) than the analogous motion in the unsubstituted molecule (8.2 kcal/mol). At the MP2 level of theory, the maximum barriers to rotation for the methyl, ethyl, isopropyl, tert-butyl, and phenyl groups are predicted to be 0.9, 6.2, 6.0, 4.6, and 2.4 kcal/mol, respectively. The results are used to benchmark the performance of the MMFF94 force field. Systematic discrepancies between MMFF94 and MP2 results were improved by modification of several torsional parameters.     

Halogen switching of azacarbenes C2NH ground states at ab initio and DFT levels

Relative stabilities and singlet–triplet energy differences are calculated for 24 C₂NX azacarbenes (where X is H, F, Cl, and Br). Three skeletal arrangements are employed including azacyclopropenylidene, [(imino)methylene]carbene, and cyanocarbene. Halogens appear to alternate the electronic ground states of C₂NH azacarbenes, from triplet to singlet states, at MP3/6‐311++G**, B1LYP/6‐311++G**, B3LYP/6‐311++G**, MP2/6‐311++G**, MP4(SDTQ)/6‐311++G**, QCISD(T)/6‐311++G**, CCSD(T)/6‐311++G**, CCSD(T)/cc‐pVTZ, G1, and G2 levels of theory. The aromatic characters of singlet cyclic azacyclopropenylidenes are measured using GIAO–NICS calculations. Linear correlations are found between the B3LYP/6‐311++G** calculated LUMO–HOMO energy gaps (ΔE_{HOMO ‐ LUMO}) of the singlet carbenes versus their corresponding singlet–triplet energy separations (ΔE). Electrophilic characters are found for all singlet azacarbenes in their addition reactions to alkenes with the highest electrophilicity being exhibited for X = F. © 2008 Wiley Periodicals, Inc. Heteroatom Chem 19:377–388, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20442     

Proton affinities of ketones, vicinal diketones and α-keto esters: a computational study

The proton affinities of seven different ketones, vicinal diketones, and α-keto esters (acetophenone, 2,2,2-trifluoroacetophenone, 2,3-butanedione, 1-phenyl-1,2-propanedione, methyl pyruvate, ethyl benzoylformate, and ketopantolactone) have been evaluated theoretically using the conventional ab initio HF and several post-HF methods (MP2, MP4, CCSD), density functional methods with the B3LYP hybrid functional, as well as some ab initio model chemistries [CBS-4M, G2(MP2), and G3(MP2)//B3LYP]. The chemical compounds studied are frequently used substrates in the asymmetric hydrogenation over chirally modified platinum catalysts where the protonation properties of the chiral modifier and the substrates are of great interest. In most cases, the proton affinities (PAs) evaluated with the CCSD/6-311+G(d,p)//B3LYP/TZVP and G2(MP2) methods are in good agreement with the existing experimental ones. However, the previously reported PA of 2,3-butanedione seems to be too high by 10-15 kJ mol⁻¹. The B3LYP/TZVP//B3LYP/TZVP and MP2/6-311+G(d,p)//B3LYP/TZVP model chemistries predict proton affinities that are systematically higher and lower than the experimental PAs, respectively. If proton affinities are evaluated as the average of the PAs calculated with these two theoretical methods a very good agreement with the experimental results is obtained. The mean absolute deviation (MAD) from experiment of this combination method for the PAs of 13 test molecules is 4.0 kJ mol⁻¹. For 9 molecules composed only of first-row atoms the MAD is 2.5 kJ mol⁻¹. The B3LYP/TZVP//B3LYP/TZVP and MP2/6-311+G(d,p)//B3LYP/TZVP methods provide significant savings in computational time and disk space compared to the CCSD/6-311+G(d,p)//B3LYP/TZVP and G2(MP2) models. Therefore, it is suggested that if no experimental or highly accurate theoretical data is available (due to computational cost), the proton affinities of similar compounds as investigated in this paper, can be evaluated with the combination method. For the studied molecules, this method gives the following PAs (in kJ mol⁻¹): 788 (2,3-butanedione, exptl 802); 798 (2,2,2-trifluoroacetophenone, exptl 799); 811 (ketopantolactone); 813 (methyl pyruvate); 825 (1-phenyl-1,2-propanedione); 862 (acetophenone, exptl 861); 865 (ethyl benzoylformate).     

Enthalpy of formation and bond energies on unsaturated oxygenated hydrocarbons using G3MP2B3 calculation methods

Enthalpies of unsaturated oxygenated hydrocarbons and radicals corresponding to the loss of hydrogen atoms from the parent molecules are intermediates and decomposition products in the oxidation and combustion of aromatic and polyaromatic species. Enthalpies (Δ_fH⁰₂₉₈) are calculated for a set of 27 oxygenated and nonoxygenated, unsaturated hydrocarbons and 12 radicals at the G3MP2B3 level of theory and with the commonly used B3LYP/6‐311g(d,p) density functional theory (DFT) method. Standard enthalpies of formation (Δ_fH⁰₂₉₈) are determined from the calculated enthalpy of reaction (ΔH⁰_rxn,298) using isodesmic work reactions with reference species that have accurately known Δ_fH⁰₂₉₈ values. The deviation between G3MP2B3 and B3LYP methods is under ±0.5 kcal mol^?1 for 9 species, 18 other species differs by less than ±1 kcal mol^?1 , and 11 species differ by about 1.5 kcal mol^?1. Under them are 11 radicals derived from the above‐oxygenated hydrocarbons that show good agreement between G3MP2B3 and B3LYP methods. G3 calculations have been performed to further validate enthalpy values, where a discrepancy of more than 2.5 kcal mol^?1 exists between the G3MP3B3 and density functional results. Surprisingly the G3 calculations support the density functional calculations in these several nonagreement cases. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 633–648, 2005     

Comparison of different ab initio theoretical models for calculating isodesmic reaction energies for small ring and related compounds

Isodesmic reactions are commonly used in ab initio studies to partially cancel errors associated with incomplete basis sets and incomplete correction for electron correlation. The errors associated with these reactions have been examined using the 6–31G^* basis set at the theoretical levels HF, MP2, MP3, MP4, and B3LYP, and using the 6–311 + G^* basis set at the HF, MP2, and B3LYP levels. As a comparison, the recently developed model chemistries, CBS-4 and CBS-Q, were also used. With hydrogenation and hydrogenolysis reactions, only the HF level gave large deviations from the experimental reaction energies. The use of hydrogen transfer reactions improved the HF calculated energies, but mixed results were obtained at the correlated levels. Some isomerization reactions and reactions of carbocations also were examined. The MP4/6–31G^* and CBS-Q levels of theory were uniformly the more satisfactory. © 1997 by John Wiley & Sons, Inc.     

Calculation of the enthalpies of formation and proton affinities of some isoquinoline derivatives

Ab initio molecular orbital theory has been used to calculate enthalpies of formation of isoquinoline, 1-hydroxyisoquinoline, 5-hydroxyisoquinoline, and 1,5-dihydroxyisoquinoline as well as some pyridine and quinoline derivatives. The proton affinities of the four isoquinoline derivatives were also obtained. The high-level composite methods G3(MP2), G3(MP2)//B3LYP, G3//B3LYP, and CBS-QB3 have been used for this study, and the results have been compared with available experimental values. For six of the eight studied compounds, the theoretical enthalpies of formation were very close to the experimental values (to within 4.3 kJ · mol⁻¹); where comparison was possible, the theoretical and experimental proton affinities were also in excellent agreement with one another. However, there is an extraordinary discrepancy between theory and experiment for the enthalpies of formation of 1-hydroxyisoquinoline and 1,5-dihydroxyisoquinoline, suggesting that the experimental values for these two compounds should perhaps be re-examined. We also show that popular low cost computational methods such as B3LYP and MP2 show very large deviations from the benchmark values.