Π-bond Order Research Articles

The hydrogen bond with carbonyl groups: theoretical study of the correlation between the X-H stretching frequency shift and the CO group proper

The empirical correlation between the stretching frequency of the carbonyl group and the shift of the v XH absorption band, proposed in the literature [3], is interpreted with a simple model, where the CO group is considered as a two π-electron system in terms of the parametric Hückel approximation. The model clearly distinguishes between the associations with OH, NH and CH groups, and offers an explanation of the different behaviour based on the properties of the donor and acceptor groups. The assumptions required for the previous simple treatment have then been tested through the analysis of a CNDO/2 treatment applied to the association of methanol with formaldehyde, acetaldehyde and acetone. After optimization of the association geometry with respect to the CO, and O-H ⋯ O bond-lengths and to the angle between the O-H and CO groups, it is found that the calculated ▵ v OH correlates with the calculated association enthalpy, transferred charge, dipole moment increment, and π-bond order, and also with the observed v CO frequency of acetaldehyde and acetone. The empirical correlation can therefore be explained in terms of both Huckel and CNDO approximations.

On the mechanism of the α-β phase transformation of quartz

With a simple geometrical model the atomic displacements at the transition from α- to β -quartz and the eigenvector of the soft mode at the Γ -point are related to a single (order-) parameter. The model is refined by taking into account possible distortions of the SiO 4 tetrahedra. These refinements, together with considerations on the temperature dependence of the order parameter, suggest an increase of π-bond order in the Si-O bond with increasing temperature as the driving mechanism for the phase transition. The model also offers a qualitative understanding of the appearance of diffuse X-ray scattering in the Γ-M direction.

INDO theoretical studies of 4-aminopyridine and protonated 4-aminopyridine

Geometry optimized INDO calculations have been performed on 4-aminopyridine (2) and protonated 4-aminopyridine (1) in order to compare their optimized geometries, π-bond orders, C(4)–NH2 rotational barriers, charge densities, and π-electron distributions. It had previously been shown that 4-aminopyridine falls far off a linear correlation of ΔG° and ΔS° for the protonation equilibria of a series of 4-substituted pyridines. The calculations support the concept that a very high rotational barrier in (1) could cause this deviation. The contribution of a quinonoid resonance hybrid (1b) to (1) was greater than (2b) to (2). The π-cloud in (1) is polarized toward the ring nitrogen, relative to (2), and the C(4)–NH2 rotational barrier in (1)(27.3 kcal mol–1) is larger than in (2)(14.7 kcal mol–1).

Theoretical study of the 4-pyridylmethyl and the 2- and 5-pyrimidyl-methyl cations and radicals

Geometry optimized SCF–MO calculations in the INDO approximation have been performed on the 4-pyridyl-methyl (1), 2-pyrimidylmethyl (3), and 5-pyrimidylmethyl (5) radicals and their corresponding cations (2), (4), and (6). The spin distribution in (1), (3), and (5) and the charge distribution in (2), (4), and (6) provide evidence that mesomeric effects in these systems are similar to those in the benzyl radical and cation. The optimized geometries all have a quinonoid distortion which is more pronounced in the cation. The π-bond orders and the ring–CH2·(or ring-CH2+) rotational barriers were obtained. The results are discussed in terms of resonance hybrid structures. The ionization potentials of the radicals were obtained and found to be in the order (3) > (1) > (5). The calculated ionization potential of (1) was 8.71 V in good agreement with the experimental value of 8.40 V obtained by electron impact measurements.

Long-range carbon-—Carbon coupling constants of 9-labeled anthracene derivatives

9- 13C-Anthrone ( 1), -9-methylanthracene ( 2), and -9-methoxyanthracene ( 3) enriched to more than 90% have been synthesized and carbon-carbon coupling constants involving the labeled-9-carbon have been determined. It is shown that these long-range carbon-carbon couplings can be used to make correct chemical shift assignments. These assignments are verified by a study of homologous compounds and of proton-coupled spectra. These long-range carbon-carbon coupling constants, along with previously obtained couplings from toluene and benzoic acid, find good correlation with the π-bond order of the respective coupling paths. A surprisingly large 4J cc is noted for 2 and 3.

?-electron structure, reactivity, and absorption spectra of anthrapyridone

The π-electronic spectra, heats of atomization, and electronic indexes of the lactam and lactim forms of anthrapyridone were calculated by the Pariser-Parr-Pople method with allowance for 25 one-electron configurations. An examination of the π-bond orders and the charges on the atoms showed that a system with a π-electron distribution close to the distribution in anthraquinone and in α-pyridone for the lactam tautometer and in α-hydroxypyridine and anthraquinone for the lactim form is formed in the lactam and lactim of anthrapyridone. The electronic absorption bands were assigned. An analysis of the calculated values demonstrated that the long-wave band in the spectrum of the anthrapyridone is due to charge transfer from the amide group to the ketone group. The reactivity and localization energy indexes of aromatic substitution reactions calculated by the Huckel MO method are in satisfactory agreement with the experimental data.

CNDO CI study of electronic structure and geometry in ground and excited states. ortho- and meta- difluorobenzenes

The modified CNDO/2 method (CNDO/s) has been applied to study the electronic structure and geometry of ortho- and meta-difluorobenzenes in their ground and first excited singlet states. Calculated density matrices have been transformed into σ- and π-bond orders by the technique suggested earlier. Bond lengths have been calculated using the Coulson bond-orderbond-length relationship and the bond angles were assumed to the 120° in both states of these molecules. The calculated rotational constants are in very good agreement with the experimental measurements recently reported by Hartford and Lombardi.

Vibrational analysis of arsenic acid and its anions

In this article we present the results of a normal coordinate analysis of arsenic acid and its anions. An exact description of the fundamentals, assigned in Part I of this series, is given. The stretching force constants, calculated by the Becher and Mattes' method are discussed, together with the π-bond order of the As-O bonds.

Cyclic inorganic compounds. Part XIV. Crystal and molecular structure of 1,cis-3,trans-5,trans-7-tetrakis(dimethylamino)-1,3,5,7-tetrafluorotetraphosphonitrile

Crystals of the title compound are monoclinic with a= 10·18 ± 0·02 b= 9·00 ± 0·02, c= 13·04 ± 0·02 A. and β= 123·1°± 0·1°, space group P21/c with Z= 2. The atomic positions have been determined by Patterson and Fourier methods and refined by least-squares to R 0·073 (1281 reflexions) from X-ray diffraction data. The molecule has a centre of symmetry and the eight-membered P4N4 ring has a chair conformation. The mean P–N ring bonds are 1·55 A and the mean P–N–P and N–P–N angles are 139·5 and 121·3°. The four phosphorus atoms are coplanar with two pairs of non-equivalent nitrogen atoms displaced 0·40 and 0·19 A from the plane. The mean P–N(amine) bond is 1·61 A implying a high π-bond order.

Zusammenhang zwischen Kernquadrupol-Kopplungskonstanten und π-Bindungsordnung ausgewählter anorganischer 2Π- und 3 Σ-Radikale / Correlation between the Nuclear Quadrupole Coupling Constants and π-bond Order of Some Inorganic 2Π and 3Σ Radicals

The nuclear quadrupole coupling constants of the 4 radicals 14NO (2Π3/2), 14NCO (2Π3/2, 14NS (2Π3/2), 33SO (3Σ-) and the 17 diamagnetic molecules 14N2O, 14N2H, 14NSF3, 14NSF, 33SCNH, 33SCO, 33SC, 33SSi, 73GeSe, 14NP, 14N≡C-H, 14N≡C-CH3, 14N≡C-CF3, 14N≡C-SiH3, 14N≡C-GeH3, 14N≡C-C-C≡C-H, 14N≡C-C≡C-CH3 have been analyzed in terms of the π-bond order. Satisfactory results are obtained when the π-bond order is expanded in powers of the ratio The validity range of this function is defined.

Spectroscopic studies on N, N-dimethylamides—II : Substituent effect on infrared carbonyl stretching vibrations and hydroxyl frequency shifts of para- and meta-substituted N, N-dimethylbenzamides and cinnamamides

Carbonyl stretching frequencies v CO and hybydroxyl frequency shifts Δ v OH of phenol as a proton donor, of series of para- and meta-substituted N, N-diniethylbenzamides and cinnamamides were measured in CCl 4. Results are correlated with the substituent constants of Swain and lopton [2] and with the carbonyl π-bond orders and the oxygen π-electron densities obtained in an H.M.O. (Ω = 1.4) calculation. Infrared frequencies are correlated with the amide rotational barriers.

Electronic Spectra and Structures of Organic π-Systems. IV. Electronic Spectra of Nitropyrroles

Abstract The electronic spectra of 2-nitropyrrole and 3-nitropyrrole have been measured in various solvents and compared with the values calculated by the variable integrals method II. Agreement between the observed and the calculated transition energies is satisfactory. From eigenvectors of both nitropyrroles, the localization of molecular orbitals to the moiety of molecule has been discussed. The π-electon densities and π-bond orders of both molecules have also been calculated.

Long-range carbon–fluorine scalar coupling in some fluorinated aromatic compounds

The proton-decoupled 13C spectrum of 2-fluoronaphthalene has been reassigned and reveals a six-bond coupling of 2·8 Hz between the fluorine and C-6; a similar coupling is observed in 4-fluorostyrene; long-range 13C–19F coupling constants in other fluorinated aromatics appear to be related to variations in the π-bond orders in the bonds between the interacting spins.

Torsional force constants and the benzene out-of-plane force field

In choosing symmetry coordinates from a redundant set of internal coordinates it is convenient to have one of the symmetrized set express the redundancy condition exactly; a mistake in the recent literature on the benzene out-of-plane field which arose from the neglect of this principle has been pointed out. A relationship between the torsional force constant, the π-bond order and the overlap integral has been developed in an attempt to reduce the uncertainty which still remains in the internal coordinate non-planar force field of benzene and it is suggested that this relationship may be useful as an aid in evaluating force fields of other molecules.

Calculation of the bond structure of phenoxymethylsilanes by the pariser-parr-pople method

Quantum chemical calculations by the Pariser-Parr-Pople method have been used to study the molecular structure of phenoxytrimethylsilanes, and the results evaluated by comparison with data from the ultraviolet spectra. Account was taken of hyperconjugation calculations from the methyl groups, but this was shown to be a negligible influence. The results show d π- p π bonding, with a π-bond order of 0.326–0.331, occurs between the silicon and oxygen atoms.

High-resolution nuclear magnetic resonance spectra of phenanthrenes—V

Abstract Complete iterative analyses have been carried out for the 60 MHz 1H NMR spectra of dibenzo[c, g]phenanthrene (I), or 3,4,5,6-dibenzophenanthrene, in solutions of carbon disulphide, [2H6]benzene, [2H5]pyridine and dimethylsulphoxide, and for the 100 MHz spectrum of benzo[g,h,i]perylene (II), or 1,12-benzoperylene, in carbon disulphide; 60 MHz spectra of I in perfluorobenzene, acetone and cyclohexane are also reported. Spin-tickling experiments at 100 MHz helped in the assignment of H-2 upfield of H-3 in I and double resonance confirmed long-range coupling of the low-field H-1 to H-5; further broadening of H-1 is attributed to weak long-range coupling to H-6 rather than to intramolecular relaxation involving H-14. Assignment of the H-5,6 AB system and the H-7 singlet in I was helped by measurements in different solvents; these also showed that v2 and v3 in the ABCD system differ least in carbon disulphide solution, and that H-1 moves downfield in benzene solution. Coupling constants, Jortho for unhindered protons in I and II may be predicted from Huckel molecular-orbital (HMO) π-bond orders via empirical equations, while C-C-H bond distortions can account for the high values of overcrowded protons. HMO calculations give fairly good agreement for chemical shifts of unhindered protons H-3 to H-7 of the planar II. An empirical parameter, φ, representing accessibility of a proton to disc-shaped molecules (with allowance for the differential effect on the reference), is lower for H-4,5 than for H-6,7 in I and suggests increased distortion of C-6,7. In [2H5]pyridine solution, all protons in I suffer downfield shifts, and in dimethylsulphoxide only the protected H-1 in I escapes large downfield shifts.

Long-range Spin–Spin Coupling in Pyrimidine Nucleosides

A study has been made of the 100 MHz p.m.r. spectra of a number of pyrimidine nucleosides in aqueous solution. Long-range five-bond spin–spin coupling interactions between the H-5 and H-1′ hydrogens were noted in uridine, 2′-deoxyuridine, 6-azauridine, and 6-azacytidine and rationalized by a predominantly anti conformation for each nucleoside. The absence of a detectable five-bond coupling in orotidine suggested that the syn conformation is preferred in this instance. The relative magnitudes of the four-bond and five-bond interactions in uridine and pseudouridine were shown to be consistent with the calculated mobile π-bond orders of the C6—N1 and C6—C5 bonds of the uracil base.

Evidence for the aromatic character of sulphonium cyclopentadienylide from nuclear magnetic resonance measurements

By analysis of the n.m.r. spectrum of dimethyl-sulphonium cyclopentadienylide (I) the π-bond orders (pr,s) were estimated and found to be within the aromatic range.

Reactions of radical anions. Part IX. The radical anion of 1-phenyl-2-trimethylsilylacetylene

The e.s.r. spectra of the radical anion of 1-phenyl-2-trimethylsilylacetylene with sodium, potassium, and caesium as gegenions in tetrahydrofuran have been obtained and analysed. Simple Huckel molecular orbital calculations were made, and coulomb and resonance integrals found which are consistent with the correct spin densities. For the carbon–silicon bond of the neutral molecule the calculated π-bond order is ca. 0·3. The radical anions are stable at –80 °C but when the temperature is raised they dimerize and the rate of dimerization has been followed in tetrahydrofuran over the temperature range –70 to –40 °C. The following values were obtained for the reaction at –60 °C: Na+ as gegenion, k= 41·1 l mol–1 s–1, ΔG‡= 10·8 kcal mol–1, ΔH‡= 9·3 kcal mol–1, ΔS‡=–6·9 cal mol–1 K–1; K+ as gegenion, k= 0·59 l mol–1 s–1, ΔG‡= 13·5 kcal mol–1, ΔH‡= 6·3 kcal mol–1, ΔS‡=–33·8 cal mol–1 K–1; Cs+ as gegenion, k= 0·26 l mol–1 s–1ΔG‡= 13·9 kcal mol–1, ΔH‡= 4·5 kcal mol–1, ΔS‡=– 44 cal mol–1 K–1.

Quantum Yield of Photo-rearrangement of Nitrones

Abstract Fifteen nitrones, N-butylidenecyclohexylamine N-oxide, N-benzylidenemethylamine N-oxide, N-diphenylmethylenemethylamine N-oxide, N-benzylidenaniline N-oxide, and its derivatives, were irradiated with light in solution, each at its longest-wavelength absorption band. Each nitrone isomerized to the corresponding oxazirane. The quantum yield of this photo-rearrangement, Φ, was measured. For each nitrone, the value of Φ is larger in cyclohexane solution than in ethanol solution. In the series of N-benzylidenaniline N-oxide and its para-substituted derivatives, the substituent lowers Φ, and the more electron-donating or the more electron-withdrawing the substituent is, the smaller is Φ. These facts are explained on the basis of a hypothesis that π should increase with the change in the π-bond order between the carbon and oxygen atoms of the nitrone group upon the lowest-energy electron excitation, ΔPC,O, calculated by the simple MO method.