The nature of the rotational barriers in simple carbonyl compounds

The rotational barriers around the CO and CC bonds are investigated in formic acid, ethanedial and glycolaldedyde molecules on the basis of DFT-B3LYP/aug-cc-pVDZ calculations. Natural bond orbitals analysis is applied to enhance physical understanding of rotational barriers. In the case of attractive barriers in formic acid and Gc-glycolaldehyde, the barrier originates from the loss of hyperconjugation that determines the equilibrium structures while for the repulsive barriers in ethanedial and Go-glycolaldehyde, both Lewis and hyperconjugation terms contribute.     

Theoretical analysis of the internal rotation and determination of molecular structures of HSSH,HSSF and FSSF

Summary A detailed investigation of the internal rotation of hydrogen persulfide and fluoro-derivatives is presented. High quality potential functions containing only three and four parameters were determined through a very simple interpolation method. Reduced torsional potentials are defined and used to assess the quality of the interpolated functions. Equilibrium structures and barriers to internal rotation reported here are in close agreement with the available experimental data. High barrier heights and the comparative analysis of structural parameters of all three molecules indicate significant bonding through the mechanism of hyperconjugation.     

Theoretical studies on the conformations of selenamides

Ab initio HF/6-31+G*, MP2/6-31+G*, B3LYP/6-31+G* level calculations have been performed on HSe-NH₂ to estimate the Se-N rotational barriers and N-inversion barriers. Two conformers have been found withsyn andanti arrangement of the NH₂ hydrogens with respect to Se-H bond. The N inversion barriers in selenamide are 1.65, 2.47, 1.93 kcal/mol and the Se-N rotational barriers are 6.58, 6.56 and 6.12 kcal/mol respectively at HF/6-31+G*, MP2/6-31+G* and B3LYP/6-31+G* levels respectively. The n_N →Σ *_Se-H negative hyperconjugation is found to be responsible for the higher rotational barriers.     

Steric strain versus hyperconjugative stabilization in ethane congeners

Rotation barriers in the group IVB ethane congeners H(3)X-YH(3) (X, Y = C, Si, Ge, Sn, Pb) have been systematically studied and deciphered using the ab initio valence bond theory in terms of the steric strain and hyperconjugation effect. Our results show that in all cases the rotation barriers are dominated by the steric repulsion whereas the hyperconjugative interaction between the X-H bond orbitals and the vicinal Y-H antibond orbitals (and vice versa) plays a secondary role, although indeed the hyperconjugation effect favors staggered structures. By the independent estimations of the hyperconjugative and steric interactions in the process of rotations, we found that the structural effect which mainly refers to the central X-Y bond relaxation makes a small contribution to the rotational barriers. Therefore, we conclude that both the rigid and fully relaxed rotations in the group IVB ethane congeners H(3)X-YH(3) observe the same mechanism which is governed by the conventional steric repulsion.     

Theoretical Studies of Inorganic Compounds. 341) Energy Decomposition Analysis of E–E Bonding in Planar and Perpendicular X2E−EX2 (E = B,Al, Ga,In, Tl; X = H,F, Cl,Br, I)

The nature of E–E bonding in group 13 compounds X₂E–EX₂ (E = B, Al, Ga, In, Tl; X = H, F, Cl, Br, I) has been investigated by means of an energy decomposition analysis (EDA) at the BP86/TZ2P level of theory. The calculated equilibrium geometries of all molecules B₂X₄?Tl₂X₄ have a perpendicular (D_2d) geometry. The largest energy barriers for rotation about the E‐E bond are predicted for the hydrogen species B₂H₄?Tl₂H₄. The EDA shows that the rotational barriers of B₂X₄?Tl₂X₄ may not be used for an estimate of the hyperconjugative strength in the D_2d structures except for the tetrahydrides. The values for the planar (D_2h) transition states reveals that π conjugation of the halogen lone‐pair electrons stabilizes the transition states. The bonding analysis shows that hyperconjugation in B₂I₄ is stronger than in B₂H₄ although the latter compound has a higher rotational barrier than the former. In B₂F₄, hyperconjugative stabilization of the perpendicular structure and conjugative stabilization of the planar structure nearly cancel each other yielding a nearly vanishing rotational barrier. The heavier analogues Al₂X₄?Tl₂X₄ have low rotational barriers and rather weak hyperconjugative interactions. The larger rotational barriers of the hydrogen systems Al₂H₄?Tl₂H₄ compared with the tetrahalogen compounds is explained with the cooperation of the relatively large hyperconjugation in the perpendicular form and the relatively weak conjugation in the planar transition structures. The EDA also indicates that the electrostatic (ΔE_elstat) and molecular orbital (ΔE_orb) components of the E–E bonding are similar in magnitude.Thecalculated B‐B bond dissociation energies of B₂X₄ (D_e = 93.0–108.4 kcal/mol) show that the bonds are rather strong. The heavier analogues Al₂X₄?Tl₂X₄ have weaker bonds (D_e = 16.6–61.7 kcal/mol). In general, the X₂E‐EX₂ bond dissociation energies follow the trend for atoms E: B ? Al > Ga > In > Tl and for atoms X: H > F > Cl > I.     

Gas-phase structures, rotational barriers, and conformational properties of hydroxyl and mercapto derivatives of cyclohexa-2,5-dienone and cyclohexa-2,5-dienthione

The rotational barriers and conformational properties of the hydroxyl and mercapto groups attached to the alpha and beta positions of cyclohexa-2,5-dione and cyclohexa-2,5-dienthione have been studied at the B3LYP/ 6-311++G(d,p) level of theory. The results show that the conformational preferences of these studied systems are the result of a subtle interplay between different competing effects (conjugation, hyperconjugation, and steric repulsions). The applicability of the density functional theory reactivity indices and the maximum hardness principle for the present systems has been analyzed.     

Probing the internal competition between alpha- and beta-elimination by fluorine substitution in adsorbed ethyl groups on Cu(100)

Key factors affecting the competition between alpha- and beta-elimination channels for adsorbed ethyl groups on a metal surface were probed by fluorine substitution. The thermal desorption products and temperatures resulting from ethyl, 2,2,2-trifluoroethyl, 1,2,2,2-tetrafluoroethyl, 1,1,2,2-tetrafluoroethyl, and pentafluoroethyl moieties adsorbed on Cu(100) provided the information about the dominant reactions and a measure of the relative rates. The alterations of number and positions of the fluorine substituents revealed that the eclipsed interactions and hyperconjugation in the transition states can determine the kinetic barriers and allowed access to the separate pathways.     

Application of 15N spectroscopy and dynamic NMR to the study of ureas,thioureas and their lewis acid adducts

Rotational barriers and ¹⁵N chemical shifts have been measured in a number of ureas and thioureas. As anticipated on the basis of the ¹⁵N shifts, several previously unobserved rotational barriers could be detected by using lanthanide reagents or a high field spectrometer. Nearly constant effects on both the rotational activation energy and the ¹⁵N shift are produced on going from ureas to the corresponding thioureas, and correlations are found between the ΔG? and δ¹⁵N values. The results are discussed in terms of lone pair delocalization, and anomalies with respect to the general behaviour are tentatively explained in the light of the effect of steric torsion in crowded structures on the ¹⁵N shifts and rotation barriers.     

Substituent effects and the role of negative hyperconjugation in siloxycarbene rearrangements

Substituent effects and the role of negative hyperconjugation in 1,2-silyl migration and decarbonylation of methoxy(substituted-siloxy)carbenes have been investigated using quantum chemical calculations and natural bond orbital analysis. It has been found that sigma-electron-withdrawing substituents generally lower the barriers for 1,2-silyl migration and decarbonylation, consistent with symmetry-forbidden concerted rearrangements involving intramolecular front-side nucleophilic attack by the carbene lone pair at silicon and by the methoxy oxygen at silicon, respectively. However, while good linear Hammett correlations are obtained for 1,2-silyl migration, those obtained for decarbonylation are poor. In addition, there appears to be a relationship between the extent of pertinent hyperconjugative interactions in the siloxycarbene conformers and the ease of intramolecular reactivity. As a matter of fact, the finding that 1,2-silyl migration is more favorable than decarbonylation seems to be primarily related to stronger negative hyperconjugation between the carbene lone pair and the O-Si antibonding orbital, compared to that between the methoxy oxygen n(sigma) lone pair and the O-Si antibonding orbital. Moreover, the activation enthalpies for 1,2-silyl migration decrease linearly with stronger negative hyperconjugation, although no such correlation could be established for decarbonylation.     

Racemization mechanisms and electronic circular dichroism of [4]heterohelicenium dyes: a theoretical study

Triarylmethylium cations with the three rings linked by two bridging groups constitute a special class of [4]heterohelicenium dyes that combine high configurational stability with the optical properties of classic dye compounds. The racemization barriers and electronic circular dichroism of seven [4]heterohelicenium analogues are investigated with density functional theory to consider different design strategies. The racemization barriers are examined with the B3LYP functional utilizing the basis set 6-31+G* with respect to bridging heteroatoms in the helicenium motif and with different bulky substituents in the helix pitch. The racemization barriers of the [4]heterohelicenium are found to be highly dependent on the relative size of the bridging atom. Nucleophilic attack at the carbenium ion leads to formation of center adducts for which the racemization barriers are found to be lowered by 8-12 kJ/mol. This finding of a nucleophilic racemization catalysis may be rationalized by the loss of conjugation upon formation of an sp(3)-hybridized carbon in the center adducts. The effect of the heteroatom substitution and center adduct formation is reflected in the electronic properties of the compound calculated with the Coulomb-attenuated method CAM-B3LYP with the basis set 6-311++G**. The excitation energies are found to be highly dependent on the twisting of the helicenium framework and only weakly influenced by the electronic nature of the bridging substituent. The electronic circular dichroism is evaluated, as the rotational strength is found to be highly dependent on both the overall molecular structure and substitution pattern but with no simple correlation between structure and circular dichroism (CD) response. The calculations reveal that the magnitude of rotational strength in most cases is dominated by the angle between the electronic and magnetic transition dipole moments. Finally, it is found that computational screening of many different structures and substituents might be needed to find target structures with maximized rotational strength.     

Microwave rotational spectra and structures of 2-fluoropyridine and 3-fluoropyridine

The ground state rotational spectra of 2-fluoropyridine and 3-fluoropyridine have been investigated using both Fourier transform microwave (FTMW) and chirped pulse Fourier transform microwave (cp-FTMW) spectroscopies. In addition to the parent species, the spectra of the (13)C and (15)N singly substituted isotopologues were recorded in the 8-23 GHz region in natural abundance. The rotational constants determined for the seven isotopologues of each were used to calculate relevant geometric parameters including the bond distances and angles of the pyridine ring backbone. The derived structures show a more pronounced deviation from the pyridine ring geometry when the fluorine substituent is ortho to nitrogen which is consistent with ab initio predictions at various levels of theory. Analysis of the (14)N hyperfine structure provided an additional source of information about the electronic structure surrounding the nitrogen atom as a function of fluorine substitution. Together, the experimental results are consistent with a bonding model that involves hyperconjugation whereby fluorine donates electron density from its lone pair into the π-system of pyridine.     

13C NMR relaxation study of molecular motions in tetraphenyltin and tetra(p-tolyl)tin in solution

The Woessner approach is applied to the 13C relaxation data for tetraphenyltin (1) and tetra(p-tolyl)tin (2) in CDCl3 solution over the temperature range 5-42 degrees C to obtain correlation times for rotational motions and hence the activation barriers. Quantum mechanical computations were carried out to obtain the rotational energy barriers for comparison. For 2 the relaxation data indicate (1) slower ring rotation than in 1, (2) highly hindered internal rotation of the methyl group. IR and chemical shift data support the hypothesis of hyperconjugation of the methyl correlated with interaction between the pi-electrons and the 5d orbitals of tin in the (p-tolyl)Sn moiety to account for the hindrances to the rotations of the ring and the methyl. The activation barrier for the tolyl group rotation is found to be much higher than that for the phenyl rotation. However, the Woessner approach yields an anomalously high barrier for the methyl rotation. An explanation based on correlated rotations of the tolyl ring and the methyl is offered.     

The gas-phase H/D exchange mechanism of protonated amino acids

A mass spectrometry and Density Functional Theory study of gas-phase H/D exchange in protonated Ala, Cys, Ile, Leu, Met, and Val is reported. Site-specific rate constants were determined and results identify the alpha-amino group as the protonation site. Lack of exchange on the Cys thiol group is explained by the absence of strong intramolecular hydrogen bonding within the reaction complex. In aliphatic amino acids the presence of a methyl group at the beta-C atom was found to lower the site-specific H/D exchange rate for amino hydrogens. Study of the exchange mechanism showed that isotopic exchange occurs in two independent reactions: in one, only the carboxylic hydrogen is exchanged and in the other, both carboxylic and amino group hydrogens exchange. The proposed reaction mechanisms, calculated structures of various species, and a number of structural findings are consistent with experimental data.     

Density functional study of the conformational space of 4C1 D-glucuronic acid

The conformational space of (4)C(1) alpha- and beta-d-glucuronic acid was scanned by HF/3-21G(p) calculations followed by optimization of the 15 most stable structures for each, using the B3LYP density functional theory method in conjunction with a diffuse polarized valence triple-zeta basis set. We found a general preference of the alpha anomers in the isolated molecules in agreement with the large endo-anomeric hyperconjugation effects in these structures. From the other intramolecular interactions (exo-anomeric hyperconjugation, hydrogen-bonding, dipole-dipole, and steric interactions), the effect of the hydrogen bonding is the most pronounced and plays a major role in determining the stability order within the alpha and beta series. The most stable conformer of both alpha and beta (4)C(1) d-glucuronic acid is the structure with the maximum number (5) of intramolecular hydrogen bonds. Introduction of solvent (water) effects by the SCI-PCM model resulted in two characteristic changes of the energetic properties: the gas-phase stability order changed considerably, and the energy range of the 15 most stable conformers decreased from 30 to 15 kJ/mol. The geometrical parameters reflect well the superimposed effects of hyperconjugation and hydrogen-bonding interactions. Most characteristics are the variations of the C-O bond distances (within a range of 0.04 A) upon the combined intramolecular effects.     

Vibrational and Thermodynamic Properties of 2,2',4,4',6,6'-Hexanitroazobenzene and Its Derivatives: A Density Functional Theory Study

HNAB (2,2′,4,4′,6,6′‐hexanitroazobenzene) and its derivatives have been optimized to obtain their molecular geometries and electronic structures by using density functional theory at the B3LYP/6‐31G* level. Their IR spectra have been computed and assigned by vibrational analysis. The strongest peaks are attributed to the N? O asymmetric stretching of nitro groups. Its central position moves towards higher frequency as the number of nitro groups increases. It is obvious that there is hydrogen‐bonding between amino and nitro groups in amino derivatives. Based on the frequencies scaled by 0.96 and the principle of statistical thermodynamics, the thermodynamic properties have been evaluated, which are linearly related with the temperature, as well as the number of nitro and amino groups, respectively, obviously showing good group additivity. And the thermodynamic functions for the nitro derivatives increase much more than those for the amino derivatives with the increase of the number of substituents. The values of heat of formation (HOF) for the nitro derivatives increase gradually with n, while those of the amino derivatives decrease smoothly with n.     

Revisiting isoreticular MOFs of alkaline earth metals: a comprehensive study on phase stability, electronic structure, chemical bonding, and optical properties of A-IRMOF-1 (A = Be, Mg, Ca, Sr, Ba)

Formation energies, chemical bonding, electronic structure, and optical properties of metal-organic frameworks of alkaline earth metals, A-IRMOF-1 (where A = Be, Mg, Ca, Sr, or Ba), have been systemically investigated with DFT methods. The unit cell volumes and atomic positions were fully optimized with the Perdew-Burke-Ernzerhof functional. By fitting the E-V data into the Murnaghan, Birch and Universal equation of states (UEOS), the bulk modulus and its pressure derivative were estimated and provided almost identical results. The data indicate that the A-IRMOF-1 series are soft materials. The estimated bandgap values are all ca. 3.5 eV, indicating a nonmetallic behavior which is essentially metal independent within this A-IRMOF-1 series. The calculated formation energies for the A-IRMOF-1 series are -61.69 (Be), -62.53 (Mg), -66.56 (Ca), -65.34 (Sr), and -64.12 (Ba) kJ mol(-1) and are substantially more negative than that of Zn-based IRMOF-1 (MOF-5) at -46.02 kJ mol(-1). From the thermodynamic point of view, the A-IRMOF-1 compounds are therefore even more stable than the well-known MOF-5. The linear optical properties of the A-IRMOF-1 series were systematically investigated. The detailed analysis of chemical bonding in the A-IRMOF-1 series reveals the nature of the A-O, O-C, H-C, and C-C bonds, i.e., A-O is a mainly ionic interaction with a metal dependent degree of covalency. The O-C, H-C, and C-C bonding interactions are as anticipated mainly covalent in character. Furthermore it is found that the geometry and electronic structures of the presently considered MOFs are not very sensitive to the k-point mesh involved in the calculations. Importantly, this suggests that sampling with Γ-point only will give reliable structural properties for MOFs. Thus, computational simulations should be readily extended to even more complicated MOF systems.     

Negative hyperconjugation in methyl and silyl anions and its effect on the acidities of CH(4-n)Fn CH(4-n)C1n SiH(4-n)Fn and SiH(4-n)Cln

Group separation reactions calculated using an ab intio molecular orbital calculation at the MP4/6-31 + + G(d,p) level of theory, show the negative hyperconjugation between fluorine atoms to be larger in methanes than in silanes. Stabilisation due to negative hyperconjugation is larger in anions than in identically substituted neutral molecules, e.g. 43.1 kcal mol⁻¹ in CF₃⁻ compared with 26.7 kcal mol⁻¹ in CHF₃. By contrast, in chloro-substituted methanes, silanes, methyl anions and silyl anions, group separation energies are approximately zero, indicating no appreciable negative hyperconjugation. An -chloro substituent is more effective than an -fluoro one at delocalising the negative charge of an anion and, as a consequence, the chloromethanes and chlorosilanes are all more acidic than the identically substitued fluoromethanes and fluorosilanes. For chloro-substituted molecules the acidity is linearly dependent on the number of chlorine atoms; for fluoro-substituted molecules stabilisation by negative hyperconjugation results in each additional fluorine atom increasing the acidity by larger increments.     

Chemical Reactivity Controlled by Negative Hyperconjugation: A Theoretical Study

Negative hyperconjugation is a general phenomenon that can be observed in many areas of chemistry. The knowledge of its impact on structural parameters and conformational issues is well established, but little is known about its importance for chemical reactivity. Here we present a systematic study of different aspects of negative hyperconjugation on the reactivity of complex heterocyclic systems using density functional theory. Intermediates from the reaction of nitrogen‐based nucleophiles with bis(1,3,4‐thiadiazolo)‐1,3,5‐triazinium halides serve as benchmark systems to demonstrate the effects of negative hyperconjugation on bond lengths, on the relative stability of conformational isomers and transition structures and, most importantly, on the different reaction pathways of these species. The computational results provided here are in part supported by experiments reported elsewhere.     

Effect of packing and tilt on the rotational barriers of an amino, nitro-substituted phenylene ethynylene trimer

Rotational potentials are computed for heptamers and nonamers of an amino, nitro-substituted phenylene ethynylene trimer molecule. A herringbone and a parallel-slipped packing arrangement are considered. The effect of tilting the molecules with respect to the surface as well as the effect of the gold support are also taken into account. The herringbone structure with the molecules perpendicular to the surface has a low rotational barrier (2 kcal/mol). Tilting the molecules by 30 degrees increases the rotational barriers significantly (16 kcal/mol). The parallel-slipped structure has rotational barriers of 7 kcal/mol. Including the effect of the gold support increases the rotational barriers for tilted molecules but has little effect on perpendicular molecules.