A dynamic nuclear magnetic resonance spectroscopic study of conformational properties of 1,3-dioxepane and 4,4,7,7-tetramethyl-1,3-dioxepane

The 251 MHz ¹H and the natural abundance 63.1 MHz ¹³C NMR spectra of 1,3-dioxepane (1) and 4,4,7,7-tetramethyl-1,3-dioxepane (2) have been investigated over the temperature range of 5 to ?180 °C. While the spectra of 1 show no dynamic NMR effect, compound 2 exists in solution as a 1:1 mixture of a symmetrical (C₂) twist-chair and its mirror image conformation. The free energy barrier for the conformational racemization of 2 is 43 kJ mol^?1 (10.3 kcal mol^?1). Interconversion paths between various conformations of 2 are discussed. Compound 1 is suggested to have a symmetrical (C₂) twist-chair conformation which is rapidly pseudorotating via a chair conformation to achieve a time averaged symmetry of C_2v, even at ?180 °C.     

Rotational barriers in bisadducts of 1-cyano-1-methylethyl radicals with nitrones and nitroso compounds

The temperature-dependent ¹H NMR spectra have been measured for the bisadducts 2a and 3 of 1-cyano-1-methylethyl radicals with α-phenyl-N-benzylnitrone [N-(benzylidene)benzylamine N-oxide] and nitrosobenzene, respectively. A free energy of activation of ΔG^≠ = 62.8 ～ 64.4 kJ mol^?1 (15.0 ～ 15.4 kcal/mol) at 24 ～ 45 °C has been obtained for 2a by applying the Eyring equation to the rate constants at the coalescence points of the methyl signals. The line shapes due to four methyl signals exchanging between two sites of equal population have been simulated by the theoretical calculation to give an activation enthalpy of 52.3 kJ mol^?1 (12.5 kcal/mol) and the corresponding entropy of ?39.3 J K^?1 mol^?1 for 3. As the inversion barriers at pyramidal nitrogens of hydroxylamine derivatives should be lowered in N-phenyl derivatives because of conjugation, the rate process which is responsible for the observed temperature dependent NMR spectra can be assigned to restricted rotation around one of the skeletal C? N? O? C bonds. The relevance of the results to the conformations of nitroxides derived from the spin trapping method using α-phenyl-N-t-butylnitrone [N-(benzylidene)-t-butylamine N-oxide] and nitroso spin traps is discussed.     

A proton nuclear magnetic resonance spectroscopic study of conformational equilibration of 3-azabicyclo[3.2.2]nonane

The 251 MHz ₁H NMR spectra of 3-azabicyclo[3.2.2]nonane (1) have been measured from ?40 to ?175°C. Below about ?140°C a conformational process which equilibrates the methylene protons of the CH₂NCH₂ moiety of 1 becomes slow on the NMR time scale. The free-energy of activation (ΔG^#) for this process is 5.9 kcal mol^?1. The results can be interpreted either in terms of ring inversion of the seven-membered ring or limited pseudorotation of the six-membered ring. Possible pathways for effecting conformational equilibration in 1 are discussed.     

The synthesis and conformational analysis of tetrahydro-1,2,4-oxadiazines

The synthesis and variable temperature ¹H and ¹³C NMR spectra of three tetrahydro-1,2,4-oxadiazines are reported. The N(4)-Me inversion barriers are 6.8–7.0 (ax→ts) and 7.4–7.9 kcal mol^?1 (eq→ts) with ΔG° 0.6–0.9 kcal mol^?1. The N(2)-Me inversion barriers are 10.4–11.4 (ax→ts) and 11.6–13.1 kcal mol^?1 (eq→ts) with ΔGδ 1.2–1.7 kcal mol^?1. The barrier to ring inversion is ca. 12.7 kcal mol^?1. “R value” analysis shows the ring to have a 56.5±2δ dihedral angle about the C(5)-(6) bond, indicative of the expected chair conformation.     

Dynamic proton magnetic resonance studies on complex spin systems. Non-mutual three-spin exchange in N-(trideuteriomethyl)-2-cyanoaziridine

At room temperature and below, the proton NMR spectrum of N-(trideuteriomethyl)-2-cyanoaziridine consists of two superimposed ABC patterns assignable to two N-invertomers; a single time-averaged ABC pattern is observed at 158.9°C. The static parameters extracted from the spectra in the temperature range from –40.3 to 23.2°C and from the high-temperature spectrum permit the calculation of the thermodynamic quantities ΔH⁰ = ?475±20 cal mol^?1 (?1.987 ± 0.084 kJ mol^?1) and ΔS⁰ = 0.43±0.08 cal mol^?1 K^?1 (1.80±0.33 J mol^?1 K^?1) for the cis ? trans equilibrium. Bandshape analysis of the spectra broadened by non-mutual three-spin exchange in the temperature range from 39.4–137.8°C yields the activation parameters ΔH_tc^≠ = 17.52±0.18 kcal mol^?1 (73.30±0.75 kJ mol^?1), ΔS_tc^≠ = ?2.08±0.50 cal mol^?1 K^?1 (?8.70±2.09 J mol^?1 K^?1) and ΔG_tc^≠ (300 K) = 18.14±0.03 kcal mol^?1 (75.90±0.13 kJ mol^?1) for the trans → cis isomerization. An attempt is made to rationalize the observed entropy data in terms of the principles of statistical thermodynamics.     

Ring and nitrogen inversion in a flexible highly symmetrical molecule: The conformations of 1,3,7,9,13,15,19,21-octaazapentacyclo[19,3,1,1,3,7,19,13,115,19]

The possible conformations of the title compound and their modes of interconversion via ring and nitrogen inversion processes are delineated. At high temperatures (> +80°) the ¹H NMR spectra are consistent with time averaged D_4th symmetry and rapid ring and nitrogen inversion. At lower temperatures (ca. ?10°) the time-averaged symmetry if D_2d and inversion of the 6-membered rings is frozen out, nitrogen inversion remaining rapid. The free energy of activation for the total inversion of all four 6-membered rings is 13·5 kcal mole ^?1, higher than in similar monocyclic systems. This higher energy is a reflection of the multiple ring inversion pathway required for total inversion of all the 6-membered rings.     

Influence of chemical exchange on the temperature dependence of Eu(fod)3-induced shifts

Temperature dependences of the paramagnetic shifts induced by Eu(fod)₃ in ¹H NMR spectra of ethylene oxide in carbon disulphide solution are obtained in the temperature range from +40 to ? 100°C at 100 MHz and from +30 to ?60°C at 60 MHz. The influence of chemical exchange leads to a decrease of the observed paramagnetic shifts with decreasing temperature. It is shown that a modified Swift and Connick equation can be used to describe the observed dependences. Upper limits of the mean lifetimes of the Eu(fod)₃-ethylene oxide adduct are τ_p < 1·7 × 10^?8 s at 14 °C and τ_p < 1 × 10^?8 s at 20 °C, respectively. The corresponding activation energy is equal to V_a = 13·7 kcal/mol.     

Synthesis and dynamic NMR studies of the 3,7-diazabicyclo[4.1.0]heptane system

A new bicyclic system, 3,7-diazabicyclo[4.1.0]heptane, has been prepared from 3-(ethoxycarbonyl)-7,3-oxazabicyclo[4.1.0]heptane by reaction with sodium azide and reduction of the resulting tosyloxy azide with lithium aluminum hydride. The molecule can exist in four stereoisomeric half-chairs, depending on the configuration of the two nitrogen atoms. Half-chair ring reversal and piperidine nitrogen inversion are fast on the NMR time scale at all observed temperatures. Inversion of the secondary aziridine nitrogen becomes slow as the temperature is lowered (T_c= ?10°C). Complete analysis of the ¹H spectrum was possible with the 1,5,5-trideuteriated analog. At slow exchange, two aziridine invertomers are present with an exo/endo ratio of approximately 0.7 in toluene-d₈, 0.7 in CH₂Cl₂ and 1.7 in CHCl₃/CH₂Cl₂. The free energy of activation for nitrogen inversion is 13.2 kcal mol^?1 at ?10°C in CHCl₃/CH₂Cl₂.     

Data on the thermochemistry of azoalkanes

The rate constant of the primary decomposition step was determined for four symmetrical and four unsymmetrical azoalkanes. From the experimental activation energies and some literature enthalpy data, the following enthalpies of formation of radicals and group contributions were calculated: ΔH_? (CH₃N₂) = 51.5 ± 1.8 kcal mol^?1, ΔH_? (C₂H₅N₂) = 44.8 ± 2.5 kcal mol^?1, ΔH_? (2?C₃H₇N₂) = 37.9 ± 2.2 kcal mol^?1, [N_A-(C)] = 27.6 ± 3.7 kcal mol^?1, [N_A-(?_A) (C)] = 61.2 ± 3.1 kcal mol^?1.     

Barriers to nitrogen inversion in 6-membered rings : Nitrogen inversion in tetrahydro-1,2-oxazines

A study of the nitrogen inversion process in the bicyclic oxazine, N-methyl-2-oxa-3-azabicyclo[2,2,2]octane, by ¹³C NMR spectrocopy reveals a free energy of activation of 14.9 kcal mole^-1. Detailed examination of the kinetics of the process observed in the ¹H spectra of N-methyl tetrahydro-1,2-oxazine shows ΔH^≠ 15.1±0.4 kcal mole^-1 and ΔS^≠ 2.3 ±1.5 cal mol^-1K^-1. It is concluded from the similarity in the activation parameters that both processes arise from nitrogen inversion.     

Atomistic understanding of the C·T mismatched DNA base pair tautomerization via the DPT: QM and QTAIM computational approaches

It was established that the cytosine·thymine (C·T) mismatched DNA base pair with cis‐oriented N1H glycosidic bonds has propeller‐like structure (|N3C4C4N3| = 38.4°), which is stabilized by three specific intermolecular interactions–two antiparallel N4H…O4 (5.19 kcal mol^?1) and N3H…N3 (6.33 kcal mol^?1) H‐bonds and a van der Waals (vdW) contact O2…O2 (0.32 kcal mol^?1). The C·T base mispair is thermodynamically stable structure (ΔG_int = ?1.54 kcal mol^?1) and even slightly more stable than the A·T Watson–Crick DNA base pair (ΔG_int = ?1.43 kcal mol^?1) at the room temperature. It was shown that the C·T ? C*·T* tautomerization via the double proton transfer (DPT) is assisted by the O2…O2 vdW contact along the entire range of the intrinsic reaction coordinate (IRC). The positive value of the Grunenberg's compliance constants (31.186, 30.265, and 22.166 Å/mdyn for the C·T, C*·T*, and TS_{C·T ? C*·T*}, respectively) proves that the O2…O2 vdW contact is a stabilizing interaction. Based on the sweeps of the H‐bond energies, it was found that the N4H…O4/O4H…N4, and N3H…N3 H‐bonds in the C·T and C*·T* base pairs are anticooperative and weaken each other, whereas the middle N3H…N3 H‐bond and the O2…O2 vdW contact are cooperative and mutually reinforce each other. It was found that the tautomerization of the C·T base mispair through the DPT is concerted and asynchronous reaction that proceeds via the TS_{C·T ? C*·T*} stabilized by the loosened N4? H? O4 covalent bridge, N3H…N3 H‐bond (9.67 kcal mol^?1) and O2…O2 vdW contact (0.41 kcal mol^?1). The nine key points, describing the evolution of the C·T ? C*·T* tautomerization via the DPT, were detected and completely investigated along the IRC. The C*·T* mispair was revealed to be the dynamically unstable structure with a lifetime 2.13·× 10^?13 s. In this case, as for the A·T Watson–Crick DNA base pair, activates the mechanism of the quantum protection of the C·T DNA base mispair from its spontaneous mutagenic tautomerization through the DPT. © 2013 Wiley Periodicals, Inc.     

Carbon-13 nuclear magnetic resonance spectra and conformations of cis,cis-1,5-cyclooctadiene monoepoxide and cis,syn,cis-1,5-cyclooctadiene diepoxide

The natural-abundance ¹³C NMR spectra of cis,cis-1,5-cyclooctadiene monoepoxide and cis,syn,cis-1,5-cyclooctadiene diepoxide have been investigated over the temperature range of – 10 to – 180°C. Whereas the spectra of the former showed no dynamic NMR effect, two different conformations in the ratio of 3:1 were observed at low temperatures for the latter. The free-energy barrier (ΔG^≠) for conversion of the major conformation to the minor conformation is calculated to be 5.9°0.2 kcal mol^?1 from a line-shape analysis of spectra obtained at intermediate temperatures. It is shown that cis,syn,cis-1,5-cyclooctadiene diepoxide exists in solution in chair (major) and in twist-boat (minor) conformations of slightly different energies. Interconversion paths between these conformations are discussed. The monoepoxide is suggested to have a twist-boat conformation that is rapidly pseudorotating via a boat conformation even at – 180°C.     

13C, 1H and 31P dynamic NMR studies of tetraalkyldiphosphanes: Barrier to internal rotation about the P?P Bond in tetraisopropyldiphosphane

The ¹H, ³¹P and ¹³C NMR spectra of tetramethyldiphosphane (1), tetraethyldiphosphane (2) and tetraisopropyldiphosphane (3) have been studied in the temperature range 30 to ?150°C and at magnetic induction up to 5.87 T. In the range ?100 to ?135 °C, the ¹H and ¹³C spectra of 3 show important changes which can be attributed to freezing the interconversion between two equivalent non-trans conformations. The line shape analysis of the ¹³C signals leads to ΔG^≠ = 29.4 kJ mol^?1 at ?113 °C for the dynamic process involved. The spin coupling parameters ¹J(PP) and N(PC) = ¹J(PC) + ²J(PC) observed for 1 in the temperature range 30 to ?120 °C show variations which could be due to a preference for the trans conformation in this diphosphane.     

Sels de Pyrylium. XVIII. Etude par RMN du Carbone-13 de Sels d'Aminopyrylium et des Barrières de Rotation dans quelques Cations N,N-Diméthylaminopyrylium

The C-2—N bond of 2-N,N-dimethylaminopyrylium cations has a partial π character due to the conjugation of the nitrogen lone-pair with the ring. The values of ΔG^≠, ΔH^≠, ΔS^≠ parameters related to the corresponding hindered rotation have been determined by ¹³C NMR total bandshape analysis. This conjugation decreases the electrophilic character of carbon C-4 so that the displacement of the alkoxy group is no longer possible. Such a hindered rotation also exists in 4-N,N-dimethylaminopyrylium cations and the corresponding ΔG^≠ parameters have been evaluated. Comparison of these two cationic species shows that hindered rotation around the C—N bond is larger in position 4 than in position 2. Furthermore, the barrier to internal rotation around the C-2? N bond decreases with increasing electron donating power of the substituent at position 4. ΔG^≠ values decreases from 19.1 kcal mol^?1 (79.9 kJ mol^?1) to 12.6 kcal mol^?1 (52.7 kJ mol^?1) according to the following sequence for the R-4 substituents: -C₆H₅, -CH₃, -OCH₃, -N(CH₃)₂.     

A dynamic nmr study of restricted rotation of two substituted phenyl groups in a c/s 1,2-diphenylcyclobutane

Dlmethyl 2,2',6.6'-tetrachloro-b-truxlnale (1) displays restricted rotation of its phenyl groups as evidenced by lineshape analysis of the dynamic¹H and ¹³C NMR spectra with the fallowing activation parameters: ΔH≠ 10.98 ± 0.24 kcal mol^-1, ΔS≠ -4.38 ± 0.99 cal mol^-1K^-1. In the transition state for rotation the two phenyl groups are perpendicular to one another and the act of rotation Involves one phenyl group at a time. The major contribution to the barrier of rotation has its origin In the chlorine atoms. X-Ray analysis of 1 yields a structure with an angle of pucker of 14.3° for the cyclobulane ring in which one chlorine group hovers over the cyclobutane rihg while the two chlorines of the other phenyl group avoid the cyclobutane ring.     

The Unexpected Mechanism Underlying the High‐Valent Mono‐Oxo‐Rhenium(V) Hydride Catalyzed Hydrosilylation of CN Functionalities: Insights from a DFT Study

In this study, we theoretically investigated the mechanism underlying the high‐valent mono‐oxo‐rhenium(V) hydride Re(O)HCl₂(PPh₃)₂ ( 1 ) catalyzed hydrosilylation of C?N functionalities. Our results suggest that an ionic S_N2‐Si outer‐sphere pathway involving the heterolytic cleavage of the Si?H bond competes with the hydride pathway involving the C?N bond inserted into the Re?H bond for the rhenium hydride ( 1 ) catalyzed hydrosilylation of the less steric C?N functionalities (phenylmethanimine, PhCH=NH, and N‐phenylbenzylideneimine, PhCH=NPh). The rate‐determining free‐energy barriers for the ionic outer‐sphere pathway are calculated to be ～28.1 and 27.6 kcal mol^?1, respectively. These values are slightly more favorable than those obtained for the hydride pathway (by ～1–3 kcal mol^?1), whereas for the large steric C?N functionality of N,1,1‐tri(phenyl)methanimine (PhCPh=NPh), the ionic outer‐sphere pathway (33.1 kcal mol^?1) is more favorable than the hydride pathway by as much as 11.5 kcal mol^?1. Along the ionic outer‐sphere pathway, neither the multiply bonded oxo ligand nor the inherent hydride moiety participate in the activation of the Si?H bond.     

Conformational Analysis. XIX properties and reactions of 1,3-oxathianes VIII A 1H NMR conformational study of methyl-substituted derivatives

The addition of thioacetic acid to unsaturated alcohols or acids was utilized to obtain mercaptoalkanols which were condensed with suitable carybonyl compounds to prepare 24 methyl-substituted 1,3-oxathianes. The ¹H NMR spectra of the 1,3-oxathiane products were recorded at 60, 100 and/or 300 MHz and fully analysed. The results are best explained by a chair form which is completely staggered in the C-4? C-5? C-6 moiety ψ₄₅ or (ψ₅₆=60±1°). 1,3-Oxathianes having syn-axial 2,4- (and/or 2,6-) methyl-methyl interactions exist appreciably, if not exclusively, in twist forms. The vicinal coupling constants lead to the conformational free energies of axial methyl groups at C-4, ΔG°=7.4±0.4 kJ mol^?1, and at C-5, ΔG°=3.7±0.3 kJ mol^?1, in good agreement with previous estimates. They also show that both r-4,cis-5,trans-6- and r-4,trans-5,trans-6- trimethyl-1,3-oxathianes greatly favour the chiar form where the methyl group at C-4 is axial. The chair-twist energy parameters are reestimated at ΔH°_CT 27.0 kJ mol^?1, ΔS°_CT 11.6J mol^?1K^?1, and ΔG°_CT(298) 23.5 kJ mol^?1 for a 2,5-twist form.     

An experimental study of some NMR methods for the measurement of slow exchange rates

The rates of interconversion of the cis and trans rotational isomers of N-trifluoroacetyl-N-methylbenzylamine have been determined by a variety of NMR lineshape experiments and by several double resonance methods. Comparison of the results for the slow exchange region suggests that a Fourier transform version of the transfer of saturation method of Forsen and Hoffman, as well as transfer of magnetization after selective 180° inversion of the resonance of one rotamer, give rate constants nearly as reliable as lineshape methods. Use of the rate of signal recovery after saturation of the resonance of one rotamer gave results which were less accurate. The available data produced the following activation parameters for amide rotation in this compound: ΔG^?, 18.9 kcal mol^?1; ΔH?, 22.3 kcal mol^?1 and ΔS?, 9.8 e.u. when the solute was 2 M in 1,1,2,2-tetrachloroethane-d₂.     

Etude structurale de la bipyrimidine par RMN en phase nématique

The 240 MHz NMR spectra of bipyrimidine in the nematic phase of p-methoxybenzylidene-p-n-butylaniline is analysed. The para H,H inter-ring distance is determined. The barrier to internal rotation of the inter-ring C? C bond is studied. Excellent agreement between experimental and calculated spectra is obtained using potential function V(?) = ½ Σ_NV_n (1 – cosn ?) with V₁ = V₂ = V₃ = 0 cal mol^?1 and V₄ = 500 cal mol^?1.     

Enriched 13C NMR of a metallated α-sulphinyl ester; temperature effect

Lithium and magnesium enolates of an α-sulphinyl ester 83% ¹³C enriched at C-1 and C-2 are studied by ¹³C NMR at different temperatures. It is shown that two metaliated species are present in the case of lithium which exchange at ?60°C, ΔG≠ ?60° = 37.8 KJ mol^?1 (9.05 Kcal mol^?1) and that there is no rapid exchange between the non-metallated ester and the metallated species (on the NMR time scale). In the case of magnesium, two or three metallated species are formed, according to the temperature which do not exchange up to 0 °C.