We present an analysis of the performances of a parameter free density functional model (PBE0) obtained combining the so called PBE generalized gradient functional with a predefined amount of exact exchange. The results obtained for structural, thermodynamic, kinetic and spectroscopic (magnetic, infrared and electronic) properties are satisfactory and not far from those delivered by the most reliable functionals including heavy parameterization. The way in which the functional is derived and the lack of empirical parameters fitted to specific properties make the PBE0 model a widely applicable method for both quantum chemistry and condensed matter physics.

Toward reliable density functional methods without adjustable parameters: The PBE0 model

In this Account we have compiled a list of reliable bond energies that are based on a set of critically evaluated experiments. A brief description of the three most important experimental techniques for measuring bond energies is provided. We demonstrate how these experimental data can be applied to yield the heats of formation of organic radicals and the bond enthalpies of more than 100 representative organic molecules.

Bond Dissociation Energies of Organic Molecules

Implicit Solvation Models: Equilibria, Structure, Spectra, and Dynamics.

Bond dissociation energies of organic molecules

The major source of error in most ab initio calculations of molecular energies is the truncation of the one‐electron basis set. Extrapolation to the complete basis set second‐order (CBS2) limit using the N−1 asymptotic convergence of N‐configuration pair natural orbital (PNO) expansions can be combined with the use of relatively small basis sets for the higher‐order (i.e., MP3, MP4, and QCI) correlation energy to develop cost effective computational models. Following this strategy, three new computational models denoted CBS‐4, CBS‐q, and CBS‐Q, are introduced. The mean absolute deviations (MAD) from experiment for the 125 energies of the G2 test set are 2.0, 1.7, and 1.0 kcal/mol, respectively. These results compare favorably with the MAD for the more costly G2(MP2), G2, and CBS‐QCI/APNO models (1.6, 1.2, and 0.5 kcal/mol, respectively). The error distributions over the G2 test set are indistinguishable from Gaussian distribution functions for all six models, indicating that the rms errors can be interpreted in the same way that experimental uncertainties are used to assess reliability.However, a broader range of examples reveals special difficulties presented by spin contamination, high molecular symmetry, and localization problems in molecules with multiple lone pairs on the same atom. These characteristics can occasionally result in errors several times the size expected from the Gaussian distributions. Each of the CBS models has a range of molecular size for which it is the most accurate computational model currently available. The largest calculations reported for these models include: The CBS‐4 heat of formation of tetranitrohydrazine (91.5±5 kcal/mol), the CBS‐4 and CBS‐q isomerization energies for the conversion of azulene to naphthalene (ΔHcalc=−35.2±1.0 kcal/mol, ΔHexp=−35.3±2.2 kcal/mol), and the CBS‐Q heat of formation of SF6 (ΔHcalc=−286.6±1.3 kcal/mol, ΔHexp=−288.3±0.2 kcal/mol). The CBS‐Q value for the dissociation energy of a C–H bond in benzene (113.1±1.3 kcal/mol) is also in agreement with the most recent experimental result (112.0±0.6 kcal/mol). The CBS‐QCI/APNO model is applicable to the prediction of the C–H bond dissociation energies for the primary (100.7±0.7 kcal/mol calc.) and secondary (97.7±0.7 kcal/mol calc., 97.1±0.4 kcal/mol exp.) hydrogens of propane.

A complete basis set model chemistry. V. Extensions to six or more heavy atoms

A flowing afterglow/selected ion flow tube instrument has been used to measure the rates of reaction of amide ion with benzene and phenide ion with ammonia C[sub 6]H[sub 6] + NH[sub 2][sup +] = C[sub 6]H[sub 5] + NH[sub 3]. The ratio of these rate constants gives a free energy change. [Delta][sub reaction]G[sub 300] = 3.58 [+-] 0.06 kcal mol[sup [minus]1]. Use of the established gas-phase acidity of ammonia gives a value for [Delta][sub acid]G[sub 300](C[sub 6]H[sub 6]) of 392.9 [+-] 0.4 kcal mol[sup [minus]1]. From the computed value for [Delta][sub acid]S[sub 300](C[sub 6]H[sub 6]) of 29.6 [+-] 1.0 cal mol[sup [minus]1] K[sup [minus]1], the enthalpy change, [Delta][sub acid]H[sub 300](C[sub 6]H[sub 6]) = 401.7 [+-] 0.5 kcal mol[sup [minus]1], is derived. The enthalpy of deprotonation of benzene, the C-H bond dissociation energy, and the electron affinity of the phenyl radical are simply related to each other. [Delta][sub acid]H[sub 300](C[sub 6]H[sub 6]) = DH[sub 300](C[sub 6]H[sub 5]-H) + IP(H) [minus] EA(C[sub 6]H[sub 5]). Since earlier photoelectron experiments have provided a value for the electron affinity for the phenyl radical, EA-(C[sub 6]H[sub 5]) = 25.3 [+-] 0.1 kcal mol[sup [minus]1] the enthalpy of deprotonation can be used to extract a value formore » the C-H bond enthalpy of benzene at 300 K and the C-H bond energy at 0 K, D[sub 0]. These bond energies are used to compute the heats of formation of the phenyl radical at 0 and 300 K. 65 refs., 3 tabs.« less

The C-H Bond Energy of Benzene

Energy-resolved collision-induced dissociation of metal carbonyl anions M(CO)[sub n][sup [minus]] (M = V, Cr, Mn, and Co) is used to determine sequential metal-carbonyl bond energies. The derived thermochemistry can be used in conjunction with literature measurements of neutral chromium carbonyl electron affinities to provide (CO)[sub n]Cr-CO bond strengths, which are compared to previous experimental determinations. Literature observations of ligand exchange reactions are used to derive other metal-ligand bond strengths. Analysis of the present data and previous values for the iron and nickel carbonyl anions shows that most metal-carbonyl ligand bond strengths are ca. 40 kcal/mol, which is apparently the intrinsic metal-carbonyl bond strength in anions. Several of the values for D[M[sup [minus]N]-2CO] are significantly lower than average; these bond strengths correlate with the energy needed to promote the metal anion into a state suitable for bonding. The electronic promotion is apparently complete after the addition of two carbonyl ligands. Comparisons of isoelectronic species confirm the general order of metal-carbonyl bond strengths, anion [>=] neutral [>=] cation. 88 refs., 6 figs., 5 tabs.

Bond strengths in first-row-metal carbonyl anions

Sequential solvation of atomic transition-metal ions. The second solvent molecule can bind more strongly than the first

Advances in flow reactor techniques for the study of gas‐phase ion chemistry

Energy-resolved collision-induced dissociation of Fe(CO) n - (n=1-4) and Ni(CO) n - (n=2, 3) is used to determine the metal-carbonyl bond energies (kcal/mol) D[(CO) 3 Fe - -CO]=41.7±2.5, D[(CO) 2 Fe - -CO]=42.4±3.5, D[(CO)Fe - -CO]=35.7±3.5, D[Fe - -CO]=33.7±3.5, D[(CO) 2 Ni - -CO]=38.5±2.3, and D[(CO)Ni - -CO]=43.4±5.8

Robert R. Squires

Papers

The C-H Bond Energy of Benzene

Bond strengths in first-row-metal carbonyl anions

Sequential solvation of atomic transition-metal ions. The second solvent molecule can bind more strongly than the first

Advances in flow reactor techniques for the study of gas‐phase ion chemistry

Metal (iron and nickel) carbonyl bond strengths in Fe(CO)n- and Ni(CO)n-