Race and ethnicity data for first, middle, and surnames.

We provide the largest compiled publicly available dictionaries of first, middle, and surnames for the purpose of imputing race and ethnicity using, for example, Bayesian Improved Surname Geocoding (BISG). The dictionaries are based on the voter files of six U.S. Southern States that collect self-reported racial data upon voter registration. Our data cover the racial make-up of a larger set of names than any comparable dataset, containing 136 thousand first names, 125 thousand middle names, and 338 thousand surnames. Individuals are categorized into five mutually exclusive racial and ethnic groups - White, Black, Hispanic, Asian, and Other - and racial/ethnic probabilities by name are provided for every name in each dictionary. We provide both probabilities of the form ℙ(race|name) and ℙ(name|race), and conditions under which they can be assumed to be representative of a given target population. These conditional probabilities can then be deployed for imputation in a data analytic task for which self-reported racial and ethnic data is not available.

Addressing census data problems in race imputation via fully Bayesian Improved Surname Geocoding and name supplements.

Prediction of individuals' race and ethnicity plays an important role in studies of racial disparity. Bayesian Improved Surname Geocoding (BISG), which relies on detailed census information, has emerged as a leading methodology for this prediction task. Unfortunately, BISG suffers from two data problems. First, the census often contains zero counts for minority groups in the locations where members of those groups reside. Second, many surnames-especially those of minorities-are missing from the census data. We introduce a fully Bayesian BISG (fBISG) methodology that accounts for census measurement error by extending the naïve Bayesian inference of the BISG methodology. We also use additional data on last, first, and middle names taken from the voter files of six Southern states where self-reported race is available. Our empirical validation shows that the fBISG methodology and name supplements substantially improve the accuracy of race imputation, especially for racial minorities.

Atmospheric formation of the NO3 radical from gas-phase reaction of HNO3 acid with the NH2 radical: proton-coupled electron-transfer versus hydrogen atom transfer mechanisms.

The gas-phase reaction of nitric acid with the amidogen radical under atmospheric conditions has been investigated using quantum mechanical (QCISD and CCSD(T)) and DFT (B3LYP, BH&HLYP, M05, M05-2X, and M06-2X) calculations with the 6-311+G(2df,2p), aug-cc-pVTZ, aug-cc-pVQZ and extrapolation to the CBS basis sets. The reaction begins with the barrierless formation of a hydrogen-bonded complex, which can undergo two different reaction pathways, in addition to the decomposition back to the reactants. The lowest energy barrier pathway involves a proton-coupled electron-transfer mechanism, whereas the highest energy barrier pathway takes place through a hydrogen atom transfer mechanism. The performance of the different DFT functionals in predicting both the geometries and relative energies of the stationary points investigated has been analyzed.

Unexpected reactivity of amidogen radical in the gas phase degradation of nitric acid.

The gas phase reaction between nitric acid and amidogen radical has been investigated employing high level quantun-mechanical electronic structure methods and variational transition state theory kinetic calculations. Our results show that the reaction proceeds through a proton coupled electron transfer mechanism with a rate constant of 1.81 × 10(-13) cm(3)·molecule(-1)·s(-1) at 298 K. This value is similar to the rate constants for the reactions of hydroxyl radical with either ammonia or nitric acid. An analysis of these data in the context of the chemistry of the atmosphere suggests that the amidogen radical, formed in the oxidation of ammonia by hydroxyl radical, reacts with nitric acid regenerating ammonia. On the basis of these findings, we propose a potential new catalytic-like cycle which couples the oxidation of ammonia by hydroxyl radical and the reaction of nitric acid with amidogen radical in the Earth's atmosphere.

The reaction of formaldehyde carbonyl oxide with the methyl peroxy radical and its relevance in the chemistry of the atmosphere.

The reaction of formaldehyde carbonyl oxide (H2COO) with the methyl peroxy radical (CH3OO), a prototype of the reactions of carbonyl oxides with alkyl peroxy radicals of potential interest in atmospheric chemistry, has been investigated by means of quantum-mechanical electronic structure methods (CASSCF, CASPT2, UQCISD, and UCCSD(T)) and DFT functionals (B3LYP, BH&HLYP, M05 and M06-2X). Two reaction paths have been found for the lowest-barrier reaction, namely the CH3OO radical addition to the carbon atom of H2COO leading to the formation of the CH3OOCH2OO radical adduct. Both pathways begin with the formation of a pre-reactive complex with binding energies of 5.39 and 5.13 kcal mol(-1). The corresponding transition states are predicted to lie 2.64 and 0.25 kcal mol(-1), respectively, below the energy of the reactants and the rate constant of the global reaction is calculated to be 3.74 × 10(-12) cm(3) molecules(-1) s(-1) at 298 K. Since the CH3OOCH2OO radical adduct is formed with an internal energy excess of about 45 kcal mol(-1), it can decompose unimolecularly into formaldehyde and the CH2(O)OOCH radical. This unimolecular decomposition involves an intramolecular H-atom transfer followed by the decomposition of the CH2OOCH2OOH radical intermediate. Kinetic calculations based on the collision-reaction master equation employing the MultiWell Program Suite reveal that the CH3OOCH2OO radical adduct is stabilized in 86.9%, whereas 13.10% of the reaction corresponds to the formation of H2CO plus the CH2(O)OOH radical. It is concluded that the methyl peroxy radical addition to substituted carbonyl oxides might be the source of low volatility oligomers observed in secondary organic aerosols in chamber studies.

On the Dissociation of Ground State trans-HOOO Radical: A Theoretical Study.

The hydrotrioxyl radical (HOOO(•)) plays a crucial role in atmospheric processes involving the hydroxyl radical (HO(•)) and molecular oxygen (O2). The equilibrium geometry of the electronic ground state (X (2)A'') of the trans conformer of HOOO(•) and its unimolecular dissociation into HO(•) (X (2)Π) and O2 (X (3)Σg(-)) have been studied theoretically using CASSCF and CASPT2 methodologies with the aug-cc-pVTZ basis set. On the one hand, CASSCF(19,15) calculations predict for trans-HOOO(•) (X (2)A'') an equilibrium structure showing a central O-O bond length of 1.674 Å and give a classical dissociation energy De = 1.1 kcal/mol. At this level of theory, it is found that the dissociation proceeds through a transition structure involving a low energy barrier of 1.5 kcal/mol. On the other hand, CASPT2(19,15) calculations predict for trans-HOOO(•) (X (2)A'') a central O-O bond length of 1.682 Å, which is in excellent agreement with the experimental value of 1.688 Å, and give De = 5.8 kcal/mol. Inclusion of the zero-point energy correction (determined from CASSCF(19,15)/aug-cc-pVTZ harmonic vibrational frequencies) in this De leads to a dissociation energy at 0 K of D0 = 3.0 kcal/mol. This value of D0 is in excellent agreement with the recent experimentally determined D0 = 2.9 ± 0.1 kcal/mol of Le Picard et al. (Science 2010, 328, 1258-1262). At the CASPT2 level of theory, we did not find for the dissociation of trans-HOOO(•) (X (2)A'') an energetic barrier other than that imposed by the endoergicity of the reaction. This prediction is in accordance with the experimental findings of Le Picard et al., indicating that the reaction of HO(•) with O2 yielding HOOO(•) is a barrierless association process.

Theoretical Mechanistic Study of the Oxidative Degradation of Benzene in the Troposphere: Reaction of Benzene-HO Radical Adduct with O2.

Competing pathways arising from the reaction of hydroxycyclohexadienyl radical (1) with O2, a key reaction in the oxidative degradation of benzene under tropospheric conditions, have been investigated by means of density functional theory (UB3LYP) and quantum-mechanical (UCCSD(T) and RCCSD(T)) electronic structure calculations. The energetic, structural, and vibrational results furnished by these calculations were subsequently used to perform conventional transition-state computations to predict the rate coefficients and evaluate the product yields. The trans stereoisomer of the peroxyl radical (4) produced by the O2 addition to position 2 of benzene ring in radical 1 is energetically more stable than the cis one, although the rate coefficients at 298 K for the formation of both isomers are predicted to be similar. The cyclization of the cis isomer of 4 to a bicyclic allyl radical (5) involves calculated barrier heights (ΔU(⧧), ΔE(⧧), ΔH(⧧), and ΔG(⧧)) significantly lower than those of the cyclization of the trans isomer of 4. This implies that the formation of the cis isomer of 4 can lead to irreversible loss of radical 1 and that the observed chemical equilibrium 1 + O2 ↔ 4 essentially involves the trans isomer of 4. Although the reaction enthalpies computed for the O2 addition to position 4 of benzene ring in radical 1, affording the cis and trans stereoisomers of a peroxyl radical (6), are similar to those for the addition to position 2, the latter addition mode is clearly preferred because it involves lower barrier heights. The barrier heights computed for the cyclization of either the cis or the trans isomers of 6 to a bicyclic radical bearing a peroxy bridge (7) are about twice those computed for the cyclization of either the cis or the trans isomers of 4. Thus, under tropospheric conditions, it is unlikely that the O2 addition to position 4 of the benzene ring in radical 1 can contribute to the formation of benzene oxidation products.

Theoretical and experimental studies on the mechanism of norbornadiene Pauson-Khand cycloadducts photorearrangement. Is there a pathway on the excited singlet potential energy surface?

The intermolecular Pauson-Khand reaction (PKR), a carbonylative cycloaddition between an alkyne and an alkene, is a convenient method to prepare cyclopentenones. Using norbornadiene as alkene, a myriad of tricyclo[5.2.1.0(2,6)]deca-4,8-dien-3-ones 1 can be easily prepared. The mechanism of the photochemical rearrangement of these adducts 1 into tricyclo[5.2.1.0(2,6)]deca-3,8-dien-10-ones 2 has been studied. The ground state (S(0)) and the three lowest excited states ((3)(pi pi*), (1)(n pi*), and (3)(n pi*)) potential energy surfaces (PESs) concerning the prototypical rearrangement of 1a (the cycloadduct of the PK carbonylative cycloaddition of norbornadiene and ethyne) to 2a have been thoroughly explored by means of CASSCF and CASPT2 calculations. From this study, two possible nonadiabatic pathways for the photochemical rearrangement arise: one starting on the (3)(pi pi*) PES and the other on the (1)(n pi*) PES. Both involve initial C-C gamma-bond cleavage of the enone, which leads to the formation of a bis-allyl or an allyl-butadienyloxyl diradical, respectively, that then decays to the S(0) PES through a (3)(pi pi*)/S(0) surface crossing or a (1)(n pi*)/S(0) conical intersection, each one lying in the vicinity of the corresponding diradical minimum. Once on the S(0) PES, the ring-closure to 2a occurs with virtually no energy barrier. The viability of both pathways was experimentally studied by means of triplet sensitization and quenching studies on the photorearrangement of the substituted Pauson-Khand cycloadduct 1b (R = TMS, R' = H) to 2b. Using high concentrations of either piperylene as a triplet quencher, or benzophenone as a triplet sensitizer, the reaction rate significantly slowed down. A Stern-Volmer type plot of product 2b concentration vs triplet quencher concentration showed an excellent linear correlation, thus indicating that only one excited state is involved in the photorearrangement. We conclude that, though there is a nonadiabatic pathway starting on the (1)(n pi*) PES, the reaction product is formed through the (3)(pi pi*) state because the energy barrier involved in the initial C-C gamma-bond cleavage of the enone is much lower in the (3)(pi pi*) PES than in the (1)(n pi*) PES.

Mechanisms for the Reactions of Hydroxyl Radicals with Acrolein: A Theoretical Study.

Three low-energy pathways for the reaction of HO(•) with acrolein, a key reaction in atmospheric environments, have been investigated by means of quantum-mechanical electronic structure methods (UQCISD and RQCISD(T)). The first step of all the reaction pathways studied involves the barrierless formation of a prereaction loosely bound complex in the entrance channel, lying a few kcal/mol below the energy of the reactants. The lowest-energy barrier pathway at 0 K is found to be the HO(•) abstraction of the aldehydic H-atom through a transition-state structure lying 1.1 kcal/mol below the energy of the reactants. The addition of HO(•) to the terminal carbon atom of the C═C double bond proceeds via a transition-state structure lying 0.7 kcal/mol below the energy of reactants at 0 K, whereas the HO(•) addition to the central carbon atom takes place via a transition-state structure lying 0.8 kcal/mol above the energy of the reactants at 0 K. On the basis of conventional transition-state theory calculations at 298 K, it is predicted that 74.5% of the HO(•) reaction with acrolein proceeds via abstraction of the aldehydic H-atom, 24.2% via HO(•) addition to the terminal carbon atom of the double bond, and 1.3% via HO(•) addition to the central carbon atom of the double bond. These results are in close agreement with available experimental data.

On the nature of the unusually long OO bond in HO(3) and HO(4) radicals.

The HO(3) and HO(4) polyoxide radicals have attracted some attention due to their potential role in ozone chemistry. Experimentally, the geometrical structure of HO(3) is known whereas that of HO(4) is not. Moreover, the existence of the latter radical has been questioned. Theoretical calculations on the two species have been reported before, showing important structural differences depending on the computational level. Both radicals present an unusually long OO bond (around 1.7-1.8 A) that can be associated with an intricate interaction between HO, or HO(2), with O(2). The nature of such interaction is investigated in detail using large scale ab initio methods (CASSCF, CASPT2, MRCI, QCISD, CCSD(T)) and density functional techniques (B3LYP) in connection with extended basis sets. Stabilization enthalpies at 298 K with respect to HO (or HO(2)) and O(2) have been computed amounting to -3.21 kcal mol(-1) for HO(3) (trans conformation) and 11.33 kcal mol(-1) for HO(4) (cis conformation). The corresponding formation enthalpies are 6.12 and 11.83 kcal mol(-1). The trans conformation of HO(4) is less stable than the cis one by 6.17 kcal mol(-1). Transition states for HO(4) dissociation and for cis/trans conversion are also described.

New insight into the gas-phase bimolecular self-reaction of the HOO radical.

The singlet and triplet potential energy surfaces (PESs) for the gas-phase bimolecular self-reaction of HOO*, a key reaction in atmospheric environments, have been investigated by means of quantum-mechanical electronic structure methods (CASSCF and CASPT2). All the reaction pathways on both PESs consist of a first step involving the barrierless formation of a prereactive doubly hydrogen-bonded complex, which is a diradical species lying about 8 kcal/mol below the energy of the reactants at 0 K. The lowest energy reaction pathway on both PESs is the degenerate double hydrogen exchange between the HOO* moieties of the prereactive complex via a double proton transfer mechanism involving an energy barrier of only 1.1 kcal/mol for the singlet and 3.3 kcal/mol for the triplet at 0 K. The single H-atom transfer between the two HOO* moieties of the prereactive complex (yielding HOOH + O2) through a pathway keeping a planar arrangement of the six atoms involves a conical intersection between either two singlet or two triplet states of A' and A" symmetries. Thus, the lowest energy reaction pathway occurs via a nonplanar cisoid transition structure with an energy barrier of 5.8 kcal/mol for the triplet and 17.5 kcal/mol for the singlet at 0 K. The simple addition between the terminal oxygen atoms of the two HOO* moieties of the prereactive complex, leading to the straight chain H2O4 intermediate on the singlet PES, involves an energy barrier of 7.3 kcal/mol at 0 K. Because the decomposition of such an intermediate into HOOH + O2 entails an energy barrier of 45.2 kcal/mol at 0 K, it is concluded that the single H-atom transfer on the triplet PES is the dominant pathway leading to HOOH + O2. Finally, the strong negative temperature dependence of the rate constant observed for this reaction is attributed to the reversible formation of the prereactive complex in the entrance channel rather than to a short-lived tetraoxide intermediate.

Mechanistic study of the CH(3)O(2)(*) + HO(2)(*) --> CH(3)O(2)H + O(2) reaction in the gas phase. computational evidence for the formation of a hydrogen-bonded diradical complex.

In an attempt to understand the mechanism of the reaction of alkylperoxy radicals with hydroperoxy radical, a key reaction in both atmospheric and combustion chemistry, the singlet and triplet potential energy surfaces (PESs) for the gas-phase reaction between CH(3)O(2)(*) and HO(2)(*) leading to the formation of CH(3)OOH and O(2) have been investigated by means of quantum-mechanical electronic structure methods (CASSCF and CASPT2). In addition, standard transition state theory calculations have been carried out with the main purpose of a qualitative description of the strong negative temperature dependence observed for this reaction. All the pathways on both the singlet and triplet PESs consist of a reversible first step involving the barrierless formation of a hydrogen-bonded pre-reactive complex, followed by the irreversible formation of products. This complex is a diradical species where the two unpaired electrons are not used for bonding and is lying about 5 kcal/mol below the energy of the reactants at 0 K. The lowest energy reaction pathway occurs on the triplet PES and involves the direct H-atom transfer from HO(2) to CH(3)O(2) in the diradical complex through a transition structure lying 3.8 kcal/mol below the energy of the reactants at 0 K. Contradicting the currently accepted interpretation of the reaction mechanism, the observed strong negative temperature dependence of the rate constant is due to the formation of the hydrogen-bonded diradical complex rather than a short-lived tetraoxide intermediate CH(3)OOOOH.

Hydrogen transfer between sulfuric acid and hydroxyl radical in the gas phase: competition among hydrogen atom transfer, proton-coupled electron-transfer, and double proton transfer.

In an attempt to assess the potential role of the hydroxyl radical in the atmospheric degradation of sulfuric acid, the hydrogen transfer between H2SO4 and HO* in the gas phase has been investigated by means of DFT and quantum-mechanical electronic-structure calculations, as well as classical transition state theory computations. The first step of the H2SO4 + HO* reaction is the barrierless formation of a prereactive hydrogen-bonded complex (Cr1) lying 8.1 kcal mol(-1) below the sum of the (298 K) enthalpies of the reactants. After forming Cr1, a single hydrogen transfer from H2SO4 to HO* and a degenerate double hydrogen-exchange between H2SO4 and HO* may occur. The single hydrogen transfer, yielding HSO4* and H2O, can take place through three different transition structures, the two lowest energy ones (TS1 and TS2) corresponding to a proton-coupled electron-transfer mechanism, whereas the higher energy one (TS3) is associated with a hydrogen atom transfer mechanism. The double hydrogen-exchange, affording products identical to reactants, takes place through a transition structure (TS4) involving a double proton-transfer mechanism and is predicted to be the dominant pathway. A rate constant of 1.50 x 10(-14) cm(3) molecule(-1) s(-1) at 298 K is obtained for the overall reaction H2SO4 + HO*. The single hydrogen transfer through TS1, TS2, and TS3 contributes to the overall rate constant at 298 K with a 43.4%. It is concluded that the single hydrogen transfer from H2SO4 to HO* yielding HSO4* and H2O might well be a significant sink for gaseous sulfuric acid in the atmosphere.

New selective haloform-type reaction yielding 3-hydroxy-2,2-difluoroacids: theoretical study of the mechanism.

Experimental results of an unprecedented haloform-type reaction in which 4-alkyl-4-hydroxy-3,3-difluoromethyl trifluoromethyl ketones undergo base-promoted selective cleavage of the CO-CF(3) bond, yielding 3-hydroxy-2,2-difluoroacids and fluoroform, are rationalized using DFT (B3LYP) calculations. The gas-phase addition of hydroxide ion to 1,1,1,3,3-pentafluoro-4-hydroxypentan-2-one (R) is found to be a barrierless process, yielding a tetrahedral intermediate (INT), involving a DeltaG(r)(298 K) of -61.4 kcal/mol. The CO-CF(3) bond cleavage in INT leads to a hydrogen-bonded [CH(3)CHOHCF(2)CO(2)H...CF(3)](-) complex by passage through a transition structure (TS1) with a DeltaG()(298 K) of 20.8 kcal/mol and a DeltaG(r)(298 K) of 9.8 kcal/mol. This complex undergoes a proton transfer between its components, yielding a hydrogen-bonded [CH(3)CHOHCF(2)CO(2)...CHF(3)](-) complex. This process has associated with it a DeltaG()(298 K) of only 3.1 kcal/mol and a DeltaG(r)(298 K) of -43.3 kcal/mol. The CO-CF(2) bond cleavage in INT leads to a hydrogen-bonded [CH(3)CHOHCF(2)...CF(3)CO(2)H](-) complex by passage through a transition structure (TS3) with a DeltaG()(298 K) of 29.2 kcal/mol and a DeltaG(r)(298 K) of 25.1 kcal/mol. The lower energy barrier found for CO-CF(3) bond cleavage in INT is ascribed to the larger number of fluorine atoms stabilizing the negative charge accumulated on the CF(3) moiety of TS1, as compared to the number of fluorine atoms stabilizing the negative charge on the CH(3)CHOHCF(2) moiety of TS3. The solvent-induced effects on the two pathways, introduced within the SCRF formalism through PCM calculations, do not reverse the predicted preference of the CO-CF(3) over the CO-CF(2) bond cleavage of R in the gas phase.

Mechanism of the hydrogen transfer from the OH group to oxygen-centered radicals: proton-coupled electron-transfer versus radical hydrogen abstraction.

High-level ab initio electronic structure calculations have been carried out with respect to the intermolecular hydrogen-transfer reaction HCOOH+.OH-->HCOO.+H(2)O and the intramolecular hydrogen-transfer reaction .OOCH2OH-->HOOCH(2)O.. In both cases we found that the hydrogen atom transfer can take place via two different transition structures. The lowest energy transition structure involves a proton transfer coupled to an electron transfer from the ROH species to the radical, whereas the higher energy transition structure corresponds to the conventional radical hydrogen atom abstraction. An analysis of the atomic spin population, computed within the framework of the topological theory of atoms in molecules, suggests that the triplet repulsion between the unpaired electrons located on the oxygen atoms that undergo hydrogen exchange must be much higher in the transition structure for the radical hydrogen abstraction than that for the proton-coupled electron-transfer mechanism. It is suggested that, in the gas phase, hydrogen atom transfer from the OH group to oxygen-centered radicals occurs by the proton-coupled electron-transfer mechanism when this pathway is accessible.

Mechanism of 1,3-migration in allylperoxyl radicals: computational evidence for the formation of a loosely bound radical-dioxygen complex.

The three pathways postulated for 1,3-migration of the peroxyl group in the allylperoxyl radical (1a), a key reaction involved in the spontaneous autoxidation of unsaturated lipids of biological importance, have been investigated by means of quantum mechanical electronic structure calculations. According to the barrier heights calculated from RCCSD(T)/6-311+G(3df,2p) energies with optimized molecular geometries and harmonic vibrational frequencies determined at the UMP2/6-311+G(3df,2p) level, the allylperoxyl rearrangement proceeds by fragmentation of 1a through a transition structure (TS1) with a calculated DeltaH++(298 K) of 21.7 kcal/mol to give an allyl radical-triplet dioxygen loosely bound complex (CX). In a subsequent step, the triplet dioxygen moiety of CX recombines at either end of the allyl radical moiety to convert the complex to the rearranged peroxyl radical (1a') or to revert to the starting peroxyl radical 1a. CX shows an electron charge transfer of 0.026 e in the direction allyl --> O(2). The dominant attractive interactions holding in association the allyl radical-triplet dioxygen pair in CX are due chiefly to dispersion forces. The DeltaH(298 K) for dissociation of CX in its isolated partners, allyl radical and triplet dioxygen, is predicted to be at least 1 kcal/mol. The formation of CX prevents the diffusion of its partners and maintains the stereocontrol along the fragmentation-recombination processes. The concerted 1,3-migration in allylperoxyl radical is predicted to take place through a five-membered ring peroxide transition structure (TS2) showing two long C-O bonds. The DeltaH++(298 K) calculated for this pathway is less favorable than the fragmentation-recombination pathway by 1.9 kcal/mol. The cyclization of 1a to give a dioxolanyl radical intermediate (2a) is found to proceed through a five-membered ring transition structure (TS3) with a calculated DeltaH++(298 K) of 33.9 kcal/mol. Thus, the sequence of ring closure 1a --> 2a and ring opening 2a --> 1a' is unlikely to play any significant role in allylperoxyl rearrangement 1a --> 1a'. In the three pathways investigated, the energy of the transition structure is predicted to be somewhat lower in either heptane or aqueous solution than in the gas phase. Although the energy lowering calculated for TS1 is smaller than the calculated for TS2 and TS3, it is very unlikely that the solvent effects may reverse the predicted preference of the fragmentation-recombination pathway over the concerted and stepwise ring closure-ring opening mechanisms.

Thermal and photochemical rearrangement of bicyclo[3.1.0]hex-3-en-2-one to the ketonic tautomer of phenol. Computational evidence for the formation of a diradical rather than a zwitterionic intermediate.

The ground state (S(0)) and lowest-energy triplet state (T(1)) potential energy surfaces (PESs) concerning the thermal and photochemical rearrangement of bicyclo[3.1.0]hex-3-en-2-one (8) to the ketonic tautomer of phenol (11) have been extensively explored using ab initio CASSCF and CASPT2 calculations with several basis sets. State T(1) is predicted to be a triplet pipi lying 66.5 kcal/mol above the energy of the S(0) state. On the S(0) PES, the rearrangement of 8 to 11 is predicted to occur via a two-step mechanism where the internal cyclopropane C-C bond is broken first through a high energy transition structure (TS1-S(0)()), leading to a singlet intermediate (10-S(0)()) lying 25.0 kcal/mol above the ground state of 8. Subsequently, this intermediate undergoes a 1,2-hydrogen shift to yield 11 by surmounting an energy barrier of only 2.7 kcal/mol at 0 K. The rate-determining step of the global rearrangement is the opening of the three-membered ring in 8, which involves an energy barrier of 41.2 kcal/mol at 0 K. This high energy barrier is consistent with the fact that the thermal rearrangement of umbellulone to thymol is carried out by heating at 280 degrees C. Regarding the photochemical rearangement, our results suggest that the most efficient route from the T(1) state of 8 to ground state 11 is the essentially barrierless cleavage of the internal cyclopropane C-C bond followed by radiationless decay to the S(0) state PES via intersystem crossing (ISC) at a crossing point (S(0)()/T(1)()-1) located at almost the same geometry as TS1-S(0)(), leading to the formation of 10-S(0)() and the subsequent low-barrier 1,2-hydrogen shift. The computed small spin-orbit coupling between the T(1) and S(0) PESs at S(0)()/T(1)()-1 (1.2 cm(-)(1)) suggests that the ISC between these PESs is the rate-determining step of the photochemical rearrangement 8 --> 11. Finally, computational evidence indicates that singlet intermediate 10-S(0)() should not be drawn as a zwitterion, but rather as a diradical having a polarized C=O bond.