Improving on equilibrium isotropic nuclear shielding constants

Isotropic nuclear shielding constants at the equilibrium molecular structure σ_eq and zero‐point vibrational corrections (ZPVCs) to σ_eq are evaluated using the B3LYP/aug‐cc‐pVTZ level of theory, as well as the KT2/aug‐cc‐pVTZ level of theory. Various scaling factors and systematic corrections are obtained by linear regression to experimental shielding constants. Comparisons of the scaled and systematically corrected equilibrium and vibrationally averaged shielding constants reveal that, at the 99% confidence level, the ZPVCs via second‐order perturbation theory do not improve the agreement of B3LYP/aug‐cc‐pVTZ and KT2/aug‐cc‐pVTZ calculated shielding constants with experiment. This holds true when the same analysis is applied to CCSD(T)/aug‐cc‐pCV[TQ]Z calculated σ_eq of Teale et al. [Journal of Chemical Physics 2013, 138, 024111]. In addition, at the 99% confidence level, B3LYP/aug‐cc‐pVTZ and KT2/aug‐cc‐pVTZ scaled and systematically corrected shielding constants are found to be statistically no different from CCSD(T)/aug‐cc‐pCV[TQ]Z calculated σ_eq. The use of scaling factors and systematic corrections could thus provide a cheaper but yet reasonably accurate alternative for the study of nuclear shielding constants of larger systems.     

IR Low‐Temperature Matrix and ab Initio Study on β‐Alanine Conformers

For the first time the argon‐matrix low‐temperature FTIR spectra of β‐alanine are recorded. They reveal a quite complicated spectral pattern which suggests the presence of several β‐alanine conformers in the matrix. To interpret the spectra, the eighteen β‐alanine conformers, stable in the gas phase, are estimated at the B3LYP and MP2 levels combined with the aug‐cc‐pVDZ. Ten low‐energy structures are reoptimized at the QCISD/aug‐cc‐pVDZ and B3LYP and MP2 levels by using the aug‐cc‐pVTZ basis sets. Assignment of the experimental spectra is undertaken on the basis of the calculated B3LYP/aug‐cc‐pVDZ anharmonic IR frequencies as well as careful estimation of the conformer population. The presence of at least three β‐alanine conformers is demonstrated. The detailed analysis of IR spectra points to the possible presence of five additional β‐alanine conformers.     

Silicon amino acids

The existence and gas phase stability of silicon analogues of three natural amino acids (i.e., silicon glycine, silicon alanine, and silicon valine) belonging to the novel class of compounds termed silicon amino acids (SiAA) are investigated theoretically on the basis of ab initio QCISD/aug‐cc‐pVTZ and MP2/aug‐cc‐pVTZ calculations. All molecules studied (in their gas phase canonical forms) are structurally comparable to their proteinogenic counterparts (i.e., glycine, l ‐alanine, and l ‐valine) and capable of forming several structural isomers as such. These higher energy isomers are characterized by small relative energies (not exceeding 4 kcal mol⁻¹). The simulated IR spectra of the Si‐Gly, Si‐Ala, and Si‐Val global minima are also presented and discussed.     

New global potential energy surface of the MgH2 system and dynamics studies of the reaction H + MgH → Mg + H2

A new global potential energy surface for the ground state of MgH₂ was constructed using the permutation invariant polynomial neural network method. About 70 000 ab initio energy points were calculated via the multi‐reference configuration interaction method method with aug‐cc‐pVTZ and aug‐cc‐pVQZ basis sets, and these points were used to construct the potential energy surface (PES). To avoid basis set superposition error, the basis set was extrapolated to the complete basis set limit using the two point energy extrapolation formula. The root mean square error of the present PES is only 8.85 meV. Initial state (v = 0, j = 0) dynamics studies were performed using the time‐dependent wave packet method with a second‐order split operator for the total angular momentum J up to a value of 50. Furthermore, the reaction probability, integral cross section, and thermal rate constant are reported and compared with available theoretical studies.     

Basis sets for the evaluation of van der Waals complex interaction energies: Ne–N2 intermolecular potential and microwave spectrum

In order to obtain efficient basis sets for the evaluation of van der Waals complex intermolecular potentials, we carry out systematic basis set studies. For this, interaction energies at representative geometries on the potential energy surfaces are evaluated using the CCSD(T) correlation method and large polarized LPol‐n and augmented polarization‐consistent aug‐pc‐2 basis sets extended with different sets of midbond functions. On the basis of the root mean square errors calculated with respect to the values for the most accurate potentials available, basis sets are selected for fitting the corresponding interaction energies and getting analytical potentials. In this work, we study the Ne–N₂ van der Waals complex and after the above procedure, the aug‐pc‐2–3321 and the LPol‐ds‐33221 basis set results are fitted. The obtained potentials are characterized by T‐shaped global minima at distances between the Ne atom and the N₂ center of mass of 3.39 Å, with interaction energies of ?49.36 cm^?1 for the aug‐pc‐2–3321 surface and ?50.28 cm^?1 for the LPol‐ds‐33221 surface. Both sets of results are in excellent agreement with the reference surface. To check the potentials further microwave transition frequencies are calculated that agree well with the experimental and the aV5Z‐33221 values. The success of this study suggests that it is feasible to carry out similar accurate calculations of interaction energies and ro‐vibrational spectra at reduced cost for larger complexes than has been possible hitherto. © 2013 Wiley Periodicals, Inc.     

Nonempirical calculations of the one‐bond 29Si–13C spin–spin coupling constants taking into account relativistic and solvent corrections

The computational study of the one‐bond ²⁹Si–¹³C spin–spin coupling constants has been performed at the second‐order polarization propagator approximation (SOPPA) level in the series of 60 diverse silanes with a special focus on the main factors affecting the accuracy of the calculation including the level of theory, the quality of the basis set, and the contribution of solvent and relativistic effects. Among three SOPPA‐based methods, SOPPA(MP2), SOPPA(CC2), and SOPPA(CCSD), the best result was achieved with SOPPA(CCSD) when used in combination with Sauer's basis set aug‐cc‐pVTZ‐J characterized by the mean absolute error of calculated coupling constants against the experiment of ca 2 Hz in the range of ca 200 Hz. The SOPPA(CCSD)/aug‐cc‐pVTZ‐J method is recommended as the most accurate and effective computational scheme for the calculation of ¹J(Si,C). The slightly less accurate but essentially more economical SOPPA(MP2)/aug‐cc‐pVTZ‐J and/or SOPPA(CC2)/aug‐cc‐pVTZ‐J methods are recommended for larger molecular systems. It was shown that solvent and relativistic corrections do not play a major role in the computation of the total values of ¹J(Si,C); however, taking them into account noticeably improves agreement with the experiment. The rovibrational corrections are estimated to be of about 1 Hz or 1–1.5% of the total value of ¹J(Si,C). Copyright © 2014 John Wiley & Sons, Ltd.     

A Theoretical Study of the Kinetics of the Hydrogen Atom Abstraction Reactions from Cyclopropane by H,O (3P), and Cl (2P3/2) Atoms and OH Radicals

The rate constants of the H‐abstraction reactions from cyclopropane by H, O (³P), Cl (²P_3/2), and OH radicals have been calculated over the temperature range of 250?2500 K using two different levels of theory. Calculations of optimized geometrical parameters and vibrational frequencies are performed using the MP2 method combined with the cc‐pVTZ basis set and the 6–311++G(d,p) basis set. Single‐point energy calculations have been carried out with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (perturbatively) electron excitations CCSD(T) using either the cc‐pVTZ, aug‐cc‐pVTZ, and aug‐cc‐pVQZ basis sets or the 6–311++G(3df,3pd) basis set. The CCSD(T) calculated potential energies have been extrapolated to the complete basis limit (CBS) limit. The Full Configuration Interaction (FCI) energies have been also estimated using the continued‐fraction approximation as proposed by Goodson (J. Chem. Phys., 2002, 116, 6948–6956). Canonical transition‐state theory combined with an Eckart tunneling correction has been used to predict the rate constants as a function of temperature using two kinetic models (direct abstraction or complex mechanism) at two levels of theory (CCSD(T)‐cf/CBS//MP2/cc‐pVTZ and CCSD(T)‐cf/6–311++G(3df,3pd)//MP2/6–311++G(d,p)). The calculated kinetic parameters are in reasonable agreement with their literature counterparts for all reactions. In the light of these trends, the use of the Pople‐style basis sets for studying the reactivity of other systems such as larger cycloalkanes or halogenated cycloalkanes is recommended because the 6–311++G(3df,3pd) basis set is less time consuming than the aug‐cc‐pVQZ basis set. Based on our calculations performed at the CCSD(T)‐cf/CBS//MP2/cc‐pVTZ level of theory, the standard enthalpy of formation at 298 K for the cyclopropyl radical has been reassessed and its value is (290.5 ± 1.6) kJ mol^?1.     

Ab Initio and Quantum Chemical Topology studies on the isomerization of HONO to HNO2. Effect of the basis set in QCT

The article focus on the isomerization of nitrous acid HONO to hydrogen nitryl HNO₂. Density functional (B3LYP) and MP2 methods, and a wide variety of basis sets, have been chosen to investigate the mechanism of this reaction. The results clearly show that there are two possible paths: 1) Uncatalysed isomerisation, trans‐HONO → HNO₂, involving 1,2‐hydrogen shift and characterized by a large energetic barrier 49.7 ÷ 58.9 kcal/mol, 2) Catalysed double hydrogen transfer process, trans‐HONO + cis‐HONO → HNO₂ + cis‐HONO, which displays a significantly lower energetic barrier in a range of 11.6 ÷ 18.9 kcal/mol. Topological analysis of the Electron Localization Function (ELF) shows that the hydrogen transfer for both studied reactions takes place through the formation of a ‘dressed’ proton along the reaction path. 1 Use of a wide variety of basis sets demonstrates a clear basis set dependence on the ELF topology of HNO₂. Less saturated basis sets yield two lone pair basins, V₁(N), V₂(N), whereas more saturated ones (for example aug‐cc‐pVTZ and aug‐cc‐pVQZ) do not indicate a lone pair on the nitrogen atom. Topological analysis of the Electron Localizability Indication (ELI‐D) at the CASSCF (12,10) confirms these findings, showing the existence of the lone pair basins but with decreasing populations as the basis set becomes more saturated (0.35e for the cc‐pVDZ basis set to 0.06e for the aug‐cc‐pVTZ). This confirms that the choice of basis set not only can influence the value of the electron population at the particular atom, but can also lead to different ELF topology. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Computational thermochemistry of glycolaldehyde

We use a variant of the focal point analysis to refine estimates of the relative energies of the four low‐energy torsional conformers of glycolaldehyde. The most stable form is the cis‐cis structure which enjoys a degree of H‐bonding from hydroxyl H to carbonyl O; here dihedral angles τ₁ (O?C? C? O) and τ₂ (C? C? O? H) both are zero. We optimized structures in both CCSD(T)/aug‐cc‐pVDZ and aug‐cc‐pVTZ; the structures agree within 0.01 Å for bond lengths and 1.0 degrees for valence angles, but the larger basis brings the rotational constants closer to experimental values. According to our extrapolation of CCSD(T) energies evaluated in basis sets ranging to aug‐cc‐pVQZ the trans‐trans form (180°, 180°) has a relative energy of 12.6 kJ/mol. The trans‐gauche conformer (160°, ±75°) is situated at 13.9 kJ/mol and the cis‐trans form (0°, 180°) at 18.9 kJ/mol. Values are corrected for zero point vibrational energy by MP2/aug‐cc‐pVTZ frequencies. Modeling the vibrational spectra is best accomplished by MP2/aug‐cc‐pVTZ with anharmonic corrections. We compute the Watsonian parameters that define the theoretical vibrational‐rotational spectra for the four stable conformers, to assist the search for these species in the interstellar medium. Six transition states are located by G4 and CBS‐QB3 methods as well as extrapolation using energies for structures optimized in CCSD(T)/aug‐cc‐pVDZ structures. We use two isodesmic reactions with two well‐established thermochemical computational schemes G4 and CBS‐QB3 to estimate energy enthalpy and Gibbs energy of formation as well as the entropy of the gas phase system. Our extrapolated electronic energies of species appearing in the isodesmic reactions produce independent values of thermodynamic quantities consistent with G4 and CBS‐QB3. © 2013 Wiley Periodicals, Inc.     

Non‐empirical calculations of NMR indirect carbon‐carbon coupling constants. Part 5: Bridged bicycloalkanes

All possible J(C,C) of the bicarbocyclic frameworks together with J(C,H) and J(H,H) at bridgeheads in the series of six bridged bicycloalkanes, bicyclo[1.1.0]butane, bicyclo[2.1.0]pentane, bicyclo[3.1.0]hexane, bicyclo[2.2.0]hexane, bicyclo[3.2.0]heptane and bicyclo[3.3.0]octane, were calculated at the SOPPA level with correlation consistent Dunning sets cc‐pVTZ‐Cs augmented with inner core s‐functions and locally dense Sauer sets aug‐cc‐pVTZ‐J augmented with tight s‐functions and rationalized in terms of the multipath coupling mechanism and hybridization effects explaining many interesting structural trends. Copyright © 2003 John Wiley & Sons, Ltd.     

Highly accurate relativistic universal Gaussian basis set for Dirac–Fock–Breit calculations

Ion mobilities of H₂O⁺ drifting in helium are calculated and compared with experiment. These calculations employ global potential energy surfaces of the H₂O⁺–He complex, which in the present case were calculated ab initio at the unrestricted MP2 level of theory using a basis set of aug‐cc‐pVTZ quality, and treating the ion as a rigid body. Details are presented of the general characteristics of both the ground and first‐excited electronic states of the complex. Although only the ground‐state surface was used for the mobility calculations, the ab initio determination of the ground state necessitated the inclusion of the first‐excited state owing to the presence of a crossing between the two. This crossing is also described. Mobilities calculated from the global surfaces are in good agreement with experiment. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

ChemInform Abstract: Smallest Deltahedra Silicon Dicarbide: C2Si32‐.

A total of 20 singlet and 36 triplet C₂Si₃^2‐ isomers are obtained by quantum chemical calculations at the B3LYP/aug‐cc‐pVTZ level.     

Computational NMR coupling constants: Shifting and scaling factors for evaluating 1JCH

Optimized shifting and/or scaling factors for calculating one‐bond carbon–hydrogen spin–spin coupling constants have been determined for 35 combinations of representative functionals (PBE, B3LYP, B3P86, B97‐2 and M06‐L) and basis sets (TZVP, HIII‐su3, EPR‐III, aug‐cc‐pVTZ‐J, ccJ‐pVDZ, ccJ‐pVTZ, ccJ‐pVQZ, pcJ‐2 and pcJ‐3) using 68 organic molecular systems with 88 ¹J_CH couplings including different types of hybridized carbon atoms. Density functional theory assessment for the determination of ¹J_CH coupling constants is examined, comparing the computed and experimental values. The use of shifting constants for obtaining the calculated coupling improves substantially the results, and most models become qualitatively similar. Thus, for the whole set of couplings and for all approaches excluding those using the M06 functional, the root‐mean‐square deviations lie between 4.7 and 16.4 Hz and are reduced to 4–6.5 Hz when shifting constants are considered. Alternatively, when a specific rovibrational contribution of 5 Hz is subtracted from the experimental values, good results are obtained with PBE, B3P86 and B97‐2 functionals in combination with HIII‐su3, aug‐cc‐pVTZ‐J and pcJ‐2 basis sets. Copyright © 2013 John Wiley & Sons, Ltd.     

Structural and vibrational analysis of the OH torsional motion in difluorohydroxyborane

In this work, we present a complete structural and vibrational analysis of the OH torsional motion in difluorohydroxyborane (BF₂OH) at the HF/aug‐cc‐pVTZ, MP2(full)/aug‐cc‐pVTZ, and CCSD/aug‐cc‐pVTZ theory levels. After full relaxation of the geometry, the equilibrium structure is found in a planar conformation of C_s symmetry. The difference in the two BF distances suggests the existence of a nonbonded interaction between the fluorine and oxygen atoms. The structural and energetic variation of BF₂OH as a function of the OH torsional angle is considered. The torsional barrier, at the CCSD/aug‐cc‐pVTZ level, and including the effect of the zero‐point energy of the remaining vibrations, is found 2,728 cm^?1. In addition, an anharmonic Hamiltonian for the OH torsional mode is presented and variationally solved. To simplify the treatment and to classify the energy levels, BF₂OH is classified under a G₄ nonrigid group accounting for the inversion symmetry of the molecule and the interchange of the fluorine atoms. The computed torsional energy levels exhibit a very small inversion splitting. The torsional spectrum is simulated considering the dipole moment components along the principal axes of inertia as an explicit function of the torsional coordinate. We observe three dominant bands in the spectrum formed by doublets corresponding to ν₉ = 0 → 1, 2 transitions. The fundamental is an a‐type, Franck–Condon, transition. This is the strongest and appears at 466.80 cm^?1 with relative intensity 0.4312. The ν₉ = 0 → 2 bands correspond to doublets of b‐ and c‐type, i.e., Herzberg–Teller transitions. These are two overlapping bands found at 890.92 and 890.94 cm^?1 with intensity 0.2207 for the b‐type band and 0.2193 for the c‐type band. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Creating σ‐Holes through the Formation of Beryllium Bonds

Through the use of ab initio theoretical models based on MP2/aug‐cc‐pVDZ‐optimized geometries and CCSD(T)/aug‐cc‐pVTZ and CCSD(T)/aug‐c‐pVDZ total energies, it has been shown that the significant electron density rearrangements that follow the formation of a beryllium bond may lead to the appearance of a σ‐hole in systems that previously do not exhibit this feature, such as CH₃OF, NO₂F, NO₃F, and other fluorine‐containing systems. The creation of the σ‐hole is another manifestation of the bond activation–reinforcement (BAR) rule. The appearance of a σ‐hole on the F atoms of CH₃OF is due to the enhancement of the electronegativity of the O atom that participates in the beryllium bond. This atom recovers part of the charge transferred to Be by polarizing the valence density of the F into the bonding region. An analysis of the electron density shows that indeed this bond becomes reinforced, but the F atom becomes more electron deficient with the appearance of the σ‐hole. Importantly, similar effects are also observed even when the atom participating in the beryllium bond is not directly attached to the F atom, as in NO₂F, NO₃F, or NCF. Hence, whereas the isolated CH₃OF, NO₂F, and NO₃F are unable to yield F ??? Base halogen bonds, their complexes with BeX₂ derivatives are able to yield such bonds. Significant cooperative effects between the new halogen bond and the beryllium bond reinforce the strength of both noncovalent interactions.     

99Tc NMR as a promising technique for structural investigation of biomolecules: theoretical studies on the solvent and thermal effects of phenylbenzothiazole complex

The phenylbenzothiazole compounds show antitumor properties and are highly selective. In this paper, the ⁹⁹Tc chemical shifts based on the (^99mTc)(CO)₃(NNO) complex conjugated to the antitumor agent 2‐(4′‐aminophenyl)benzothiazole are reported. Thermal and solvent effects were studied computationally by quantum‐chemical methods, using the density functional theory (DFT) (DFT level BPW91/aug‐cc‐pVTZ for the Tc and BPW91/IGLO‐II for the other atoms) to compute the NMR parameters for the complex. We have calculated the ⁹⁹Tc NMR chemical shifts of the complex in gas phase and solution using different solvation models (polarizable continuum model and explicit solvation). To evaluate the thermal effect, molecular dynamics simulations were carried, using the atom‐centered density matrix propagation method at the DFT level (BP86/LanL2dz). The results highlight that the ⁹⁹Tc NMR spectroscopy can be a promising technique for structural investigation of biomolecules, at the molecular level, in different environments. Copyright © 2014 John Wiley & Sons, Ltd.     

Ab initio calculations of the Ar–ethane intermolecular potential energy surface using bond function basis sets

The intermolecular potential energy surface (PES) of argon with ethane has been studied by ab initio calculations at the levels of second‐order Møller–Plesset perturbation (MP2) theory and coupled‐cluster theory with single, double, and noniterative triple configurations (CCSD(T)) using a series of augmented correlation‐consistent basis sets. Two sets of bond functions, bf1 (3s3p2d) and bf2 (6s6p4d2f), have been added to the basis sets to show a dramatic and systematic improvement in the convergence of the entire PES. The PES of Ar–ethane is characterized by a global minimum at a near T‐shaped configuration with a well depth of 0.611 kcal mol^?1, a second minimum at a collinear configuration with a well depth of 0.456 kcal mol^?1, and a saddle point connecting the two minima. It is shown that an augmented correlation‐consistent basis set with a set of bond functions, either bf1 or bf2, can effectively produce results equivalent to the next larger augmented correlation‐consistent basis set, that is, aug‐cc‐pVDZ‐bf1 ≈ aug‐cc‐pVTZ, aug‐cc‐pVTZ‐bf1 ≈ aug‐cc‐pVQZ. Very importantly, the use of bond functions improves the PES globally, resulting accurate potential anisotropy. Finally, MP2 method is inadequate for accurate calculations, because it gives a potentially overestimated well depth and, more seriously, a poor potential anisotropy. © 2012 Wiley Periodicals, Inc.     

Basis set error estimation for DFT calculations of electronic g‐tensors for transition metal complexes

We present a detailed study of the basis set dependence of electronic g‐tensors for transition metal complexes calculated using Kohn–Sham density functional theory. Focus is on the use of locally dense basis set schemes where the metal is treated using either the same or a more flexible basis set than used for the ligand sphere. The performance of all basis set schemes is compared to the extrapolated complete basis set limit results. Furthermore, we test the performance of the aug‐cc‐pVTZ‐J basis set developed for calculations of NMR spin‐spin and electron paramagnetic resonance hyperfine coupling constants. Our results show that reasonable results can be obtain when using small basis sets for the ligand sphere, and very accurate results are obtained when an aug‐cc‐pVTZ basis set or similar is used for all atoms in the complex. © 2014 Wiley Periodicals, Inc.     

Highly accurate benchmark calculations of the interaction energies in the complexes C6H6···C6X6 (X = F,Cl, Br,and I)

Different from the case of the benzene dimer, the differences between the interaction energies are always less than 0.50 kcal/mol for face‐to‐face eclipsed, face‐to‐face staggered, and parallel‐displaced configurations of all investigated complexes C₆H₆···C₆X₆ (X = F, Cl, Br, and I). Hence, it is a great challenge for quantum chemists to accurately calculate the interaction energies for the three configurations of the complexes C₆H₆···C₆X₆. This work demonstrates that results obtained with the PBE0 density functional combined with the D3 dispersion correction (PBE0‐D3) and the basis set def2‐TZVPP are in excellent agreement with the estimates of the coupled‐cluster singles, doubles, and perturbative triples [CCSD(T)] complete basis set (CBS) limit. The other finding in this study is that, in comparison with the gold‐standard CCSD(T)/CBS benchmark, the spin‐component scaled (SCS) zeroth‐order symmetry‐adapted perturbation theory (SAPT0), when paired with the basis set aug‐cc‐pVDZ, performs also very well, and its performance is even better than that of the PBE0‐D3/def2‐TZVPP method or the conventional SAPT/aug‐cc‐pVQZ method. The findings of this study are very significant because both PBE0‐D3/def2‐TZVPP and SCS‐SAPT0/aug‐cc‐pVDZ can deal with the systems with more than 200 atoms.     

Theoretical study on the gas phase reaction of propargyl alcohol with hydroxyl radical

The reaction of propargyl alcohol with hydroxyl radical has been studied extensively at CCSD(T)/aug‐cc‐pVTZ//MP2/cc‐pVTZ level. This is the first time to gain a conclusive insight into the reaction mechanism and kinetics for this important reaction in detail. Two reaction mechanisms were revealed, namely addition/elimination and hydrogen abstraction mechanism. The reaction mechanism confirms that OH addition to C?C triple bond forms the chemically activated adducts, IM1 (·CHCOHCH₂OH) and IM2 (CHOH·CCH₂OH), and the hydrogen abstraction pathways (? CH₂OH bonded to the carbon atom and alcohol hydrogen) may occur via low barriers. Harmonic model of Rice–Ramsperger–Kassel–Marcus theory and variational transition state theory are used to calculate the overall and individual rate constants over a wide range of temperatures and pressures. The calculated rate constants are in good agreement with the experimental data. At atmospheric pressure with Ar as bath gas, IM1 (·CHCOHCH₂OH) and IM2 (CHOH·CCH₂OH) formed by collisional stabilization are dominant in the low temperature range. The production of CHCCHOH + H₂O via hydrogen abstraction becomes dominate at higher temperature. The fraction of IM3 (CH₂COHCH₂·O) is very significant over the moderate temperature range. © 2014 Wiley Periodicals, Inc.