Calculations on 1,8-naphthoquinone predict that the ground state of this diradical is a singlet.

(12/12)CASPT2, (16/14)CASPT2, B3LYP, and CCSD(T) calculations have been carried out on 1,8-Naphthoquinone (1,8-NQ), to predict the low-lying electronic states and their relative energies in this non-Kekulé quinone diradical. CASPT2 predicts a 1 A1 ground state, with three other electronic states-3 B2 , 3 B1 , and 1 B1 -within about 10 kcal/mol of the ground state in energy. On the basis of the results of these calculations, it is predicted that NIPES experiments on 1,8-NQ •- will find that 1,8-NQ is a diradical with a singlet ground state. © 2018 Wiley Periodicals, Inc.

Negative Ion Photoelectron Spectroscopy Confirms the Prediction of a Singlet Ground State for the 1,8-Naphthoquinone Diradical.

Negative ion photoelectron (NIPE) spectra, with 193, 266, 300, and 355 nm photons, of the radical anion of 1,8-naphthoquinone (1,8-NQ•-) have been obtained at 20 K. The electron affinity of 1,8-NQ is determined from the first resolved peak in the NIPE spectrum to be 2.965 ± 0.005 eV. Franck-Condon factors (FCFs), calculated from the CASPT2/aug-cc-pVDZ optimized geometries, normal modes, and vibrational frequencies, successfully simulate the intensity and frequencies of the spectral features that are associated with the lowest two electronic states. The NIPE spectra of 1,8-NQ•- and the peak assignments, based on the computed FCFs, confirm the theoretical predictions that 1A1 is the ground state of 1,8-NQ and 3B2 is the first excited state. The spectra provide an experimental value of Δ EST = -0.6 kcal/mol, which is 2 kcal/mol smaller in magnitude than the (12/12)CASPT2/aug-cc-pVTZ calculated value of Δ EST = -2.6 kcal/mol.

Negative Ion Photoelectron Spectroscopy Confirms the Prediction of the Relative Energies of the Low-Lying Electronic States of 2,7-Naphthoquinone.

Cryogenic negative ion photoelectron (NIPE) spectra of the radical anion of 2,7-naphthoquinone (NQ•-) have been taken at 20 K, using 193, 240, 266, 300, and 355 nm lasers for electron detachment. The electron affinity of the NQ diradical is determined from the first resolved peak in the NIPE spectrum to be 2.880 ± 0.010 eV. CASPT2/aug-cc-pVDZ calculations predict with reasonable accuracy the positions of the 0-0 bands in the three lowest electronic states of NQ. In addition, the Franck-Condon factors calculated from the CASPT2/aug-cc-pVDZ optimized geometries, vibrational frequencies, and normal modes successfully simulate the vibrational structures in these bands. The NIPE spectrum of NQ•- confirms that, as predicted, 3B2 is the ground state, and the 1B2 and 1A1 states are, respectively, 12.7 and 16.4 kcal/mol higher in energy than the triplet ground state. The experimental value of Δ EST = 12.7 kcal/mol in NQ and the finding that 1B2 is the lower energy of the two singlet states confirm the results of the previous calculations on NQ. These calculations predicted an increase in Δ EST on the substitution of both methylene groups in 2,7-naphthoquinodimethane (NQDM) by oxygens in NQ, thus providing a dramatic contrast to the decrease of 17.5 kcal/mol in Δ EST found for substitution of one methylene group by one oxygen on going from trimethylenemethane (TMM) to oxyallyl (OXA).

A Joint Experimental and Computational Study of the Negative Ion Photoelectron Spectroscopy of the 1-Phospha-2,3,4-triazolate Anion, HCPN3(.).

We report here the results of a combined experimental and computational study of the negative ion photoelectron spectroscopy (NIPES) of the recently synthesized, planar, aromatic, HCPN3(-) ion. The adiabatic electron detachment energy of HCPN3(-) (electron affinity of HCPN3(•)) was measured to be 3.555 ± 0.010 eV, a value that is intermediate between the electron detachment energies of the closely related (CH)2N3(-) and P2N3(-) ions. High level electronic structure calculations and Franck-Condon factor (FCF) simulations reveal that transitions from the ground state of the anion to two nearly degenerate, low-lying, electronic states, of the neutral HCPN3(•) radical are responsible for the congested peaks at low binding energies in the NIPE spectrum. The best fit of the simulated NIPE spectrum to the experimental spectrum indicates that the ground state of HCPN3(•) is a 5π-electron (2)A″ π radical state, with a 6π-electron, (2)A', σ radical state being at most 1.0 kcal/mol higher in energy.

Negative Ion Photoelectron Spectroscopy Confirms the Prediction that 1,2,4,5-Tetraoxatetramethylenebenzene Has a Singlet Ground State.

The negative ion photoelectron (NIPE) spectrum of 1,2,4,5-tetraoxatetramethylenebenzene radical anion (TOTMB(•-)) shows that, like the hydrocarbon, 1,2,4,5-tetramethylenebenzene (TMB), the TOTMB diradical has a singlet ground state and thus violates Hund's rule. The NIPE spectrum of TOTMB(•-) gives a value of -ΔEST = 3.5 ± 0.2 kcal/mol for the energy difference between the singlet and triplet states of TOTMB and a value of EA = 4.025 ± 0.010 eV for the electron affinity of TOTMB. (10/10)CASPT2 calculations are successful in predicting the singlet-triplet energy difference in TOTMB almost exactly, giving a computed value of -ΔEST = 3.6 kcal/mol. The same type of calculations predict -ΔEST = 6.1-6.3 kcal/mol in TMB. Thus, the calculated effect of the substitution of the four oxygens in TOTMB for the four methylene groups in TMB is very unusual, since the singlet state is selectively destabilized relative to the triplet state. The reason why TMB → TOTMB is predicted to result in a decrease in the size of -ΔEST is discussed.

Theoretical Analysis of the Fragmentation of (CO)5: A Symmetry-Allowed Highly Exothermic Reaction that Follows a Stepwise Pathway.

B3LYP and CCSD(T) calculations, using an aug-cc-pVTZ basis set, have been carried out on the fragmentation of 1,2,3,4,5-cyclopentanepentone, (CO)(5), to five molecules of CO. Although this reaction is calculated to be highly exothermic and is allowed to be concerted by the Woodward-Hoffmann rules, our calculations find that the D(5h) energy maximum is a multidimensional hilltop on the potential energy surface. This D(5h) hilltop is 16-20 kcal/mol higher in energy than a C(2) transition structure for the endothermic cleavage of (CO)(5) to (CO)(4) + CO and 11-15 kcal/mol higher than a C(s) transition structure for the loss of two CO molecules. The reasons for the very high energy of the D(5h) hilltop are discussed, and the geometries of the two lower energy transition structures are rationalized on the basis of mixing of the e(2)' HOMO and the a(2)″ LUMO of the hilltop.

Nonheme Fe(IV) Oxo Complexes of Two New Pentadentate Ligands and Their Hydrogen-Atom and Oxygen-Atom Transfer Reactions.

Two new pentadentate {N5} donor ligands based on the N4Py (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) framework have been synthesized, viz. [N-(1-methyl-2-benzimidazolyl)methyl-N-(2-pyridyl)methyl-N-(bis-2-pyridyl methyl)amine] (L(1)) and [N-bis(1-methyl-2-benzimidazolyl)methyl-N-(bis-2-pyridylmethyl)amine] (L(2)), where one or two pyridyl arms of N4Py have been replaced by corresponding (N-methyl)benzimidazolyl-containing arms. The complexes [Fe(II)(CH3CN)(L)](2+) (L = L(1) (1); L(2) (2)) were synthesized, and reaction of these ferrous complexes with iodosylbenzene led to the formation of the ferryl complexes [Fe(IV)(O)(L)](2+) (L = L(1) (3); L(2) (4)), which were characterized by UV-vis spectroscopy, high resolution mass spectrometry, and Mössbauer spectroscopy. Complexes 3 and 4 are relatively stable with half-lives at room temperature of 40 h (L = L(1)) and 2.5 h (L = L(2)). The redox potentials of 1 and 2, as well as the visible spectra of 3 and 4, indicate that the ligand field weakens as ligand pyridyl substituents are progressively substituted by (N-methyl)benzimidazolyl moieties. The reactivities of 3 and 4 in hydrogen-atom transfer (HAT) and oxygen-atom transfer (OAT) reactions show that both complexes exhibit enhanced reactivities when compared to the analogous N4Py complex ([Fe(IV)(O)(N4Py)](2+)), and that the normalized HAT rates increase by approximately 1 order of magnitude for each replacement of a pyridyl moiety; i.e., [Fe(IV)(O)(L(2))](2+) exhibits the highest rates. The second-order HAT rate constants can be directly related to the substrate C-H bond dissociation energies. Computational modeling of the HAT reactions indicates that the reaction proceeds via a high spin transition state.

The negative ion photoelectron spectrum of meta-benzoquinone radical anion (MBQ˙⁻): a joint experimental and computational study.

Negative ion photoelectron (NIPE) spectra of the radical anion of meta-benzoquinone (MBQ, m-OC6H4O) have been obtained at 20 K, using both 355 and 266 nm lasers for electron photodetachment. The spectra show well-resolved peaks and complex spectral patterns. The electron affinity of MBQ is determined from the first resolved peak to be 2.875 ± 0.010 eV. Single-point, CASPT2/aug-cc-pVTZ//CASPT2/aug-cc-pVDZ calculations predict accurately the positions of the 0-0 bands in the NIPE spectrum for formation of the four lowest electronic states of neutral MBQ from the (2)A2 state of MBQ(•-). In addition, the Franck-Condon factors that are computed from the CASPT2/aug-cc-pVDZ optimized geometries, vibrational frequencies, and normal mode vectors, successfully simulate the intensities and frequencies of the vibrational peaks in the NIPE spectrum that are associated with each of these electronic states. The successful simulation of the NIPE spectrum of MBQ(•-) allows the assignment of (3)B2 as the ground state of MBQ, followed by the (1)B2 and (1)A1 electronic states, respectively 9.0 ± 0.2 and 16.6 ± 0.2 kcal/mol higher in energy than the triplet. These experimental energy differences are in good agreement with the calculated values of 9.7 and 15.7 kcal/mol. The relative energies of these two singlet states in MBQ confirm the previous prediction that their relative energies would be reversed from those in meta-benzoquinodimethane (MBQDM, m-CH2C6H4CH2).

The negative ion photoelectron spectrum of cyclopropane-1,2,3-trione radical anion, (CO)3(•-)--a joint experimental and computational study.

Negative ion photoelectron (NIPE) spectra of the radical anion of cyclopropane-1,2,3-trione, (CO)3(•-), have been obtained at 20 K, using both 355 and 266 nm lasers for electron photodetachment. The spectra show broadened bands, due to the short lifetimes of both the singlet and triplet states of neutral (CO)3 and, to a lesser extent, to the vibrational progressions that accompany the photodetachment process. The smaller intensity of the band with the lower electron binding energy suggests that the singlet is the ground state of (CO)3. From the NIPE spectra, the electron affinity (EA) and the singlet-triplet energy gap of (CO)3 are estimated to be, respectively, EA = 3.1 ± 0.1 eV and ΔEST = -14 ± 3 kcal/mol. High-level, (U)CCSD(T)/aug-cc-pVQZ//(U)CCSD(T)/aug-cc-pVTZ, calculations give EA = 3.04 eV for the (1)A1' ground state of (CO)3 and ΔEST = -13.8 kcal/mol for the energy gap between the (1)A1' and (3)A2 states, in excellent agreement with values from the NIPE spectra. In addition, simulations of the vibrational structures for formation of these states of (CO)3 from the (2)A2″ state of (CO)3(•-) provide a good fit to the shapes of broad bands in the 266 nm NIPE spectrum. The NIPE spectrum of (CO)3(•-) and the analysis of the spectrum by high-quality electronic structure calculations demonstrate that NIPES can not only access and provide information about transition structures but NIPES can also access and provide information about hilltops on potential energy surfaces.

Heavy-atom tunneling in the ring opening of a strained cyclopropene at very low temperatures.

The highly strained 1H-bicyclo[3.1.0]-hexa-3,5-dien-2-one 1 is metastable, and rearranges to 4-oxacyclohexa-2,5-dienylidene 2 in inert gas matrices (neon, argon, krypton, xenon, and nitrogen) at temperatures as low as 3 K. The kinetics for this rearrangement show pronounced matrix effects, but in a given matrix, the reaction rate is independent of temperature between 3 and 20 K. This temperature independence means that the activation energy is zero in this temperature range, indicating that the reaction proceeds through quantum mechanical tunneling from the lowest vibrational level of the reactant. At temperatures above 20 K, the rate increases, resulting in curved Arrhenius plots that are also indicative of thermally activated tunneling. These experimental findings are supported by calculations performed at the CASSCF and CASPT2 levels by using the small-curvature tunneling (SCT) approximation.

Why is (SiO)(4) calculated to be tetrahedral, whereas (CO)(4) is square planar? A molecular orbital analysis.

Qualitative molecular orbital (MO) theory predicts that square-planar tetrasilacyclobutanetetraone D4h-(SiO)4 should, like D4h-(CO)4, have a triplet ground state, and the results of the (U)CCSD(T)-F12b/cc-pVTZ-F12//(U)B3LYP/6-311+G(2df) calculations, reported here, confirm this expectation. Calculations at the same level of theory find that square-planar tetrasilacyclobutanetetrathione D4h-(SiS)4 also has a triplet ground state. However, these ab initio calculations predict that (SiO)4 and (SiS)4 both have a singlet state of much lower energy, with a tetrahedral (Td) equilibrium geometry and six, electron-deficient, Si-Si bonds. In contrast, the lowest singlet state of (CO)4 and of (CS)4 is calculated to prefer a D4h to a Td geometry. An analysis, based on the second-order Jahn-Teller effect, rationalizes the influence of the electronegativity difference between A and Y in (AY)4 on the energy difference between a D4h and Td geometry. This analysis predicts that (BF)4 and (BCl)4, which are isoelectronic with, respectively, (CO)4 and (CS)4, should both prefer a Td to a D4h equilibrium geometry. These qualitative predictions have been confirmed by our calculations, and (BCl)4 is known experimentally to have a Td equilibrium geometry.

Negative ion photoelectron spectroscopy confirms the prediction that (CO)5 and (CO)6 each has a singlet ground state.

Cyclobutane-1,2,3,4-tetraone has been both predicted and found to have a triplet ground state, in which a b2g σ molecular orbital (MO) and an a2u π MO are each singly occupied. In contrast, (CO)5 and (CO)6 have each been predicted to have a singlet ground state. These predictions have been tested by generating the (CO)5(•-) and (CO)6(•-) radical anions in the gas phase, using electrospray vaporization of solutions of, respectively, the croconate (CO)5(2-) and rhodizonate (CO)6(2-) dianions. The negative ion photoelectron (NIPE) spectrum of the (CO)5(•-) radical anion gives an electron affinity of EA = 3.830 eV for formation of the singlet ground state of (CO)5. The triplet is found to be higher in energy by 0.850 eV (19.6 kcal/mol). The NIPE spectrum of the (CO)6(•-) radical anion gives EA = 3.785 eV for forming the singlet ground state of (CO)6, with the triplet state higher in energy by 0.915 eV (21.1 kcal/mol). (RO)CCSD(T)/aug-cc-pVTZ//(U)B3LYP/6-311+G(2df) calculations give EA values that are only approximately 1 kcal/mol lower than those measured and ΔE(ST) values that are 2-3 kcal/mol higher than those obtained from the NIPE spectra. Calculations of the Franck-Condon factors for transitions from the ground state of each radical anion, (CO)n(•-) to the lowest singlet and triplet states of the n = 4-6 neutrals, nicely reproduce all of the observed vibrational features in the low-binding energy regions of all three NIPE spectra. Thus, the calculations of both the energies and vibrational structures of the two lowest energy bands in each of the NIPE spectra support the interpretation of the spectra in terms of a singlet ground state for (CO)5 and (CO)6 but a triplet ground state for (CO)4.

Calculations on tunneling in the reactions of noradamantyl carbenes.

Noradamantylchlorocarbene has been found experimentally to undergo ring expansion to 2-chloroadamantene at cryogenic temperatures. The rate constant, calculated with inclusion of small-curvature tunneling, is within a factor of 2 of the rate constant measured at 9 K in a nitrogen matrix. Our calculations predict that noradamantylfluorocarbene will not be found to rearrange under these conditions. The rate constant for carbon tunneling in the ring expansion of noradamantylmethylcarbene (1d) to 2-methyladamantene at T

How to make the σ0π2 singlet the ground state of carbenes.

Successful strategies have previously been developed to stabilize the σ(2)π(0) singlet states of carbenes, relative to σ(1)π(1) triplet states. However, little or no attention has been paid to the stabilization of the σ(0)π(2) singlet states. We present two simple strategies to stabilize the σ(0)π(2) singlet states of carbenes, relative to both the σ(2)π(0) singlet and σ(1)π(1) triplet states. These strategies consist of destabilization of the carbene σ orbital by two, adjacent, sp(2) nitrogen lone pairs of electrons and stabilization of the carbene 2p-π orbital by incorporating it into a five-membered ring, containing two double bonds, or into a six-membered ring, containing two double bonds and a sixth atom that has a low-lying empty π orbital. B3LYP, CASPT2, and CCSD(T) calculations have been performed in order to assess the success of these strategies in creating derivatives of cyclopenta-2,4-dienylidene and cyclohexa-2,5-dienylidene with σ(0)π(6) singlet ground states. Differences between the calculated geometries and binding energies of the Xe complexes of the σ(0)π(6) singlet ground state of 2,5-diazacyclopentadienylidene (5) and the σ(2)π(0) singlet states of CH2 and CF2 are discussed.

Like (CO)4, Do (CS)4 and (CSe)4 have a triplet ground state?

Cyclobutane-1,2,3,4-tetraone, (CO)4, was computationally predicted and, subsequently, experimentally confirmed to have a triplet ground state, in which a b2g σ MO and an a2u π MO were each singly occupied. In contrast, the (U)CCSD(T) calculations reported herein found that cyclobutane-1,2,3,4-tetrathione, (CS)4, and cyclobutane-1,2,3,4-tetraselenone, (CSe)4, both had singlet ground states, in which the b2g σ MO was doubly occupied and the a2u π MO was empty. Our calculations showed that both the longer C=X distances and smaller coefficients on the carbon atoms in the b2g and a2u MOs of (CS)4 and (CSe)4 contributed to the difference between the ground states of these two molecules and the ground state of (CO)4. An experimental test of the prediction of a singlet ground state for (CS)4 is proposed.

The ground state of (CS)4 is different from that of (CO)4: an experimental test of a computational prediction by negative ion photoelectron spectroscopy.

Cyclobutane-1,2,3,4-tetrathione, (CS)4, has recently been calculated to have a singlet ground state, (1)A1g, in which the highest b2g σ MO is doubly occupied and the lowest a2u π MO is empty. Thus, (CS)4 is predicted to have a different ground state than its lighter congener, (CO)4, which has a triplet ground state, (3)B1u, in which these two MOs are each singly occupied. Here, we report the results of a negative ion photoelectron spectroscopy (NIPES) study of the radical anion (CS)4(•-), designed to test the prediction that (CS)4 has a singlet ground state. The NIPE spectrum reveals that (CS)4 does, indeed, have a singlet ground state with electron affinity (EA) = 3.75 eV. The lowest triplet state is found to lie 0.31 eV higher in energy than the ground state, and the open-shell singlet is 0.14 eV higher in energy than the triplet state. Calculations at the (U)CCSD(T)/aug-cc-pVTZ//(U)B3LYP/6-311+G(2df) level support the spectral assignments, giving EA = 3.71 eV and ΔEST = 0.44 eV. These calculated values are, respectively, 0.04 eV (0.9 kcal/mol) smaller and 0.13 eV (3.0 kcal/mol) larger than the corresponding experimental values. In addition, RASPT2 calculations with various active spaces and basis sets converge on a (1)B1u-(3)B1u energy gap of 0.137 eV, in excellent agreement with the 0.14 eV energy difference obtained from the NIPE spectrum. Finally, calculations of the Franck-Condon factors for transitions from the ground state of (CS)4(•-) to the ground ((1)A1g) and two excited states ((3)B1u, (1)B1u) of (CS)4 account for all of the major spectral peaks and nicely reproduce the vibrational structure observed in each electronic transition. The close correspondence between the calculated and the observed features in the NIPE spectrum of (CS)4(•-) provides unequivocal proof that (CS)4, unlike (CO)4, has a singlet ground state.

Synchronized aromaticity as an enthalpic driving force for the aromatic Cope rearrangement.

We report herein experimental and theoretical evidence for an aromatic Cope rearrangement. Along with several successful examples, our data include the first isolation and full characterization of the putative intermediate that is formed immediately after the initial [3,3] sigmatropic rearrangement. Calculations at the B3LYP/6-31G(d) level of theory predict reaction energy barriers in the range 22-23 kcal/mol for the [3,3]-rearrangement consistent with the exceptionally mild reaction conditions for these reactions. The experimental and computational results support a significant enthalpic contribution of the concomitant pyrazole ring formation that serves as both a kinetic and thermodynamic driving force for the aromatic Cope rearrangement.

Molecular orbitals of the oxocarbons (CO)n, n = 2-6. Why does (CO)4 have a triplet ground state?

Cyclobutane-1,2,3,4-tetrone has been both predicted and found to have a triplet ground state, in which a b(2g) σ MO and an a(2u) π MO are each singly occupied. The nearly identical energies of these two orbitals of (CO)(4) can be attributed to the fact that both of these MOs are formed from a bonding combination of C-O π* orbitals in four CO molecules. The intrinsically stronger bonding between neighboring carbons in the b(2g) σ MO compared to the a(2u) π MO is balanced by the fact that the non-nearest-neighbor, C-C interactions in (CO)(4) are antibonding in b(2g), but bonding in a(2u). Crossing between an antibonding, b(1g) combination of carbon lone-pair orbitals in four CO molecules and the b(2g) and a(2u) bonding combinations of π* MOs is responsible for the occupation of the b(2g) and a(2u) MOs in (CO)(4). A similar orbital crossing occurs on going from two CO molecules to (CO)(2), and this crossing is responsible for the triplet ground state that is predicted for (CO)(2). However, such an orbital crossing does not occur on formation of (CO)(2n+1) from 2n + 1 CO molecules, which is why (CO)(3) and (CO)(5) are both calculated to have singlet ground states. Orbital crossings, involving an antibonding, b(1), combination of lone-pair MOs, occur in forming all (CO)(2n) molecules from 2n CO molecules. Nevertheless, (CO)(6) is predicted to have a singlet ground state, in which the b(2u) σ MO is doubly occupied and the a(2u) π MO is left empty. The main reason for the difference between the ground states of (CO)(4) and (CO)(6) is that interactions between 2p AOs on non-nearest-neighbor carbons, which stabilize the a(2u) π MO in (CO)(4), are much weaker in (CO)(6), due to the much larger distances between non-nearest-neighbor carbons in (CO)(6) than in (CO)(4).

Calculations of the effects of methyl groups on the energy differences between cyclooctatetraene and bicyclo[4.2.0]octa-2,4,7-triene and between their iron tricarbonyl complexes.

In accord with experiment, DFT calculations find that cyclooctatetraene (COT, 1a) is lower in energy than its valence isomer, bicyclo[4.2.0]octa-2,4,7-triene (BCOT, 3a) and that the iron tricarbonyl complex of COT [COT-Fe(CO)(3), 2a] is lower in energy than the iron tricarbonyl complex of BCOT [BCOT-Fe(CO)(3), 4a]. Also in agreement with experiment are the DFT findings that 1,3,5,7-tetramethylCOT (TMCOT, 1b) is lower in energy than 1,3,5,7-tetramethylBCOT (TMBCOT, 3b), but that the iron tricarbonyl complex of TMCOT [TMCOT-Fe(CO)(3), 2b] is higher in energy than the iron tricarbonyl complex of TMBCOT [TMBCOT-Fe(CO)(3), 4b]. Calculations of the energies of isodesmic reactions allow the effect of each of the four methyl groups in 1b-4b to be analyzed in terms of its additive contribution to the relative energies of TMCOT (1b) and TMBCOT (3b) and to the Fe(CO)(3) binding energies in TMCOT-Fe(CO)(3) (2b) and TMBCOT-Fe(CO)(3) (4b). Our calculations also predict that the eight methyl groups in octamethylCOT-Fe(CO)(3) [OMCOT-Fe(CO)(3), 2c] should have much more than twice the effect of the four methyl groups in TMCOT-Fe(CO)(3) (2b) on raising the energy of OMCOT-Fe(CO)(3) (2c), relative to that of OMBCOT-Fe(CO)(3) (4c). The effects of the interactions between the methyl groups in OMCOT-Fe(CO)(3) (2c) and OMBCOT-Fe(CO)(3) (4c) are dissected and discussed.

Effects of geminal methyl groups on the tunnelling rates in the ring opening of cyclopropylcarbinyl radical at cryogenic temperature.

CVT + SCT calculations on the rate of tunnelling at 20 K in the ring opening of cyclopropylcarbinyl radical, substituted with geminal methyl groups at a ring carbon (1b), have been performed. The calculations predict that, contrary to expectations based on the effect of mass on the rate of tunnelling, the geminal methyl substituents in 1b should make the rate of ring opening to 1,1-dimethyl-3-butenyl radical (2b) 10(4) times faster than the rate of ring opening of unsubstituted cyclopropylcarbinyl radical (1a) to 3-butenyl radical (2a) and almost 10(6) times faster than the rate of ring opening of 1b to 2,2-dimethyl-3-butenyl radical (2c). The reasons for these unexpected findings are discussed.