Intramolecular Dynamic Processes Research Articles

A2BC-type meso-imidazolylporphyrins – New class of prospective polyfunctional molecules

A convenient synthetic strategy for the efficient preparation of new functionalized low-symmetry A2BC-type porphyrins bearing meso-areneimidazolyl fragment is developed. The reactivity of the key precursors, namely 5-bromo-15-imidazolylporphyrins, in C–C-coupling reactions was evaluated providing access to target functional derivatives. Two types of electron-withdrawing groups, namely cyano- and phenylcarboxy-ones, were introduced to the meso-position, opposite to the heterocyclic substituent. It allowed preparation of prospective push-pull derivatives along with the introduction of latent anchoring groups to the macrocycle. The peculiarities of the mutual intramolecular dynamic processes in the prepared compounds were revealed and investigated by means of variable-temperature NMR. DFT calculations of the structure, molecular orbitals and rotation activation barriers were performed, that revealed the conformational dependence of the orbitals localization. Altogether the obtained results form the basis for the rational design of highly unsymmetrical and polarized porphyrin derivatives with tunable properties.

In situ simultaneous electrochemical ESR study of radicals generated from 2,2-dinitroethene-1,1-diamine (FOX-7). Intramolecular chemical exchange resulting in an alternation line-width effect

The molecule of 2,2-dinitroethene-1,1-diamine (FOX-7) is one of the most interesting molecules with multiple redox centres stabilized by push-pull effect. To reveal the detailed mechanism of its electrochemical process radical intermediates formed in the course of its electrochemical reduction in organic aprotic media have been studied by in situ simultaneous electrochemical ESR measurements (SEESR). The radical generated on the second reduction step possesses an alternating line-width (AL) effect in the ESR spectra as a result of intramolecular dynamic processes in the timescale of ESR splitting constants. The spectra measured at different temperatures (230–335 K) were analysed with the help of a fitting program which includes a molecular dynamics. Observed dynamics describes well an asymmetric 2-site exchange model for the whole temperature range. With help of the optimized parameters and quantum chemical calculations this radical has been identified as 2,2-dinitroethane-1-amine-1-imine radical dianion, [(H2N)(HN)C=C(NO2)2]2−. The dynamic process responsible for the AL effect consists of mutual turning (changing of dihedral angle) of the both nitro groups, resulting in an intramolecular spin-density (electron) transfer. The dynamic parameters of the process have been established.

The Problem of Dynamic Process Manifestation in Identification of Organic Compounds by NMR Spectroscopy

The number, shape and position of NMR spectral lines depend on dynamic processes, and this creates certain difficulties in identification of pharmaceutical substances by NMR spectroscopy. The aim of the paper was to study instances of manifestation of intramolecular dynamic processes that affect identification of organic compounds by NMR, and to illustrate the potential of the methods used for their reduction, as well as associated problems.Materials and methods: 1H and 13C spectra of the following pharmaceutical substances: «buserelin acetate», «valsartan», «goserelin acetate», «iopromide», «clopidogrel hydrogensulfate», «omeprazole», «proroxan», «risperidone», «triptorelin acetate», and «enalapril maleate» were used to demonstrate negative effects of dynamic processes. The spatial structures of conformers were established by 1H-1H ROESY experiments. The quantum-chemical calculation of geometric and thermodynamic characteristics of different conformers was carried out by the PM3 method, and electronic characteristics—by the AM1 method with the help of the HyperChem software.Results: the authors analysed intramolecular dynamic processes which are most commonly encountered in expert work: pyramidal inversion of nitrogen in a heterocyclic compound (risperidone, proroxan, clopidogrel), rotation of molecular fragments around the amide bond (valsartan, iopromide, enalapril), prototropic rearrangements (buserelin, goserelin, omeprazole, triptorelin). The change in exchange rates was explained from the perspective of the change in the system of intra- and intermolecular nonvalent interactions.Conclusions: the use of traditional methods for increasing the rate of dynamic processes (increasing the temperature and changing the solvent) does not always eliminate the negative effects of intramolecular transformations. Methods of smoothing the spectral manifestations of dynamic processes have limited application due to strong intramolecular nonvalent interactions which prevent the conversion of the dynamic process rate into fast exchange. Experts and manufacturers should take into account the manifestation of dynamic processes during identification of pharmaceutical substances by NMR spectroscopy.

Pt–M Complexes (M=Ag, Au) as Models for Intermediates in Transmetalation Processes

Heteropolynuclear complexes [(CNC)(PPh3 )PtM(PPh3 )]ClO4 [M=Au (1), Ag (2)] and [{Pt(CNC)(PPh3 )}2 M]ClO4 [M=Ag (3), Au (4); CNC=2,6-diphenylpyridinate] were prepared and studied by X-ray crystallography, spectroscopic techniques, and DFT calculations. The X-ray crystal structures of 1, 3, and 4 confirmed the existence of Pt-M bonds and M⋅⋅⋅Cipso interactions involving one of the phenyl fragments of CNC. Their NMR spectra showed the persistence of the Pt-M interactions in solution and also revealed an intramolecular metronome-like dynamic process consisting of back-and-forth motion of the acidic M fragments along the C-Pt-C axis. DFT calculations on these complexes identified two main orbital interactions between the [PtCNC] and [M]+ fragments, namely, donation from the former to a vacant orbital of the latter and much weaker backdonation from the acidic M to the Pt fragment. Overall, the strength of the [Pt]⋅⋅⋅M interactions is higher for the gold compounds than for their silver counterparts. The interaction between the acidic center (silver or gold) and the carbon atom of one of the phenylene rings in these heteropolymetallic complexes can be envisaged as the first step in a process of interchange of aryl ligands. However, the ligand exchange cannot progress further due to the polydentate nature of the CNC ligand, and therefore these structures can be considered as frozen snapshots of a transmetalation reaction that has been arrested at different stages of the process.

Variable-Temperature Tip-Enhanced Raman Spectroscopy of Single-Molecule Fluctuations and Dynamics.

Structure, dynamics, and coupling involving single-molecules determine function in catalytic, electronic or biological systems. While vibrational spectroscopy provides insight into molecular structure, rapid fluctuations blur the molecular trajectory even in single-molecule spectroscopy, analogous to spatial averaging in measuring large ensembles. To gain insight into intramolecular coupling, substrate coupling, and dynamic processes, we use tip-enhanced Raman spectroscopy (TERS) at variable and cryogenic temperatures, to slow and control the motion of a single molecule. We resolve intrinsic line widths of individual normal modes, allowing detailed and quantitative investigation of the vibrational modes. From temperature dependent line narrowing and splitting, we quantify ultrafast vibrational dephasing, intramolecular coupling, and conformational heterogeneity. Through statistical correlation analysis of fluctuations of individual modes, we observe rotational motion and spectral fluctuations of the molecule. This work demonstrates single-molecule vibrational spectroscopy beyond chemical identification, opening the possibility for a complete picture of molecular motion ranging from femtoseconds to minutes.

On the role of HNS and HSN as light-sensitive NO-donors for delivery in biological media.

Results are presented that suggest that thiazyl hydride (HSN)/thionitrosyl hydride (sulfimide, HNS) can be used as light-sensitive compounds for NO-delivery in biological media, as well as markers for the possible detection of intermediates in nitrites + H2S reactions at the cellular level. They are expected to be more efficient than the HNO/HON isovalent species and hence they should be considered instead. A set of characteristic spectroscopic features are identified that could aid in the possible detection of these species in the gas phase or in biological environments. The possibility of intramolecular dynamical processes involving excited states that are capable of interconverting HNS and its isomeric form HSN is examined.

Collective relaxation dynamics and crystallization kinetics of the amorphous Biclotymol antiseptic

We employ dielectric spectroscopy to monitor the relaxation dynamics and crystallization kinetics of the Biclotymol antiseptic in its amorphous phase. The glass transition temperature of the material as determined by dielectric spectroscopy is Tg=290±1K. The primary (α) relaxation dynamics is observed to follow a Vogel–Fulcher–Tammann temperature dependence, with a kinetic fragility index m=86±13, which classifies Biclotymol as a relatively fragile glass former. A secondary relaxation is also observed, corresponding to an intramolecular dynamic process of the non-rigid Biclotymol molecule. The crystallization kinetics, measured at four different temperatures above the glass transition temperature, follows an Avrami behavior with exponent virtually equal to n=2, indicating one-dimensional crystallization into needle-like crystallites, as experimentally observed, with a time-constant nucleation rate. The activation barrier for crystallization is found to be Ea=115±22kJmol−1.

Phenylene ethynylene-tethered perylene bisimide folda-dimer and folda-trimer: investigations on folding features in ground and excited states.

In this work, we have elucidated in detail the folding properties of two perylene bisimide (PBI) foldamers composed of two and three PBI units, respectively, attached to a phenylene ethynylene backbone. The folding behaviors of these new PBI folda-dimer and trimer have been studied by solvent-dependent UV/Vis absorption and 1D and 2D NMR spectroscopy, revealing facile folding of both systems in tetrahydrofuran (THF). In CHCl3 the dimer exists in extended (unfolded) conformation, whereas partially folded conformations are observed in the trimer. Temperature-dependent (1) H NMR spectroscopic studies in [D8 ]THF revealed intramolecular dynamic processes for both PBI foldamers due to, on the one hand, hindered rotation around CN imide bonds and, on the other hand, backbone flapping; the latter process being energetically more demanding as it was observed only at elevated temperature. The structural features of folded conformations of the dimer and trimer have been elucidated by different 2D-NMR spectroscopy (e.g., ROESY and DOSY) in [D8 ]THF. The energetics of folding processes for the PBI dimer and trimer have been assessed by calculations applying various methods, particularly the semiempirical PM6-DH2 and the more sophisticated B97D approach, in which relevant dispersion corrections are included. These calculations corroborate the results of NMR spectroscopic studies. Folding features in the excited states of these PBI foldamers have been characterized by using time-resolved fluorescence and transient absorption spectroscopy in THF and CHCl3 , exhibiting similar solvent-dependent behavior as observed for the ground state. Interestingly, photoinduced electron transfer (PET) process from electron-donating backbone to electron-deficient PBI core for extended, but not for folded, conformations was observed, which can be explained by a fast relaxation of excited PBI stacks in the folded conformation into fluorescent excimer states.

Analysis of Intramolecular Dynamic Processes in Enantiomeric Diaryl Atropisomers and Related Derivatives by 2H NMR Spectroscopy in Polypeptide Liquid Crystals

We demonstrate the analytical potential of 2H-{1H} NMR spectroscopy in weakly ordering, chiral lyotropic liquid crystals made of poly(gamma-benzyl-L-glutamate) (PBLG) dissolved in chloroform or dichloromethane for investigating the intramolecular dynamic processes of four deuterated diaryls (derivatives of 1-(4'-methylphenyl)naphthalene). When the rotation of the aryl groups about the sp(2)--sp(2) bond is sufficiently slow relative to the NMR timescale, the method allows the spectral discrimination of enantiomeric atropisomers or enantiotopic directions in the prochiral derivatives. The effect of the position of substituents on the phenyl group on the conformational dynamics of these compounds has been examined as well as the nature of the organic co-solvent. When coalescence phenomena are observed, simulation of the experimental 2H-{1H} lineshapes using a formalism tailored for two deuterons undergoing mutual exchange allows the rate constants and the activation parameters for the internal rotation processes to be calculated. Experimental values of DeltaH(not equal) have been compared with data evaluated by molecular modelling calculations and the activation parameters are discussed for the various compounds. It is shown that these polypeptide mesophases have no significant impact on the interconversion dynamics of these compounds. In contrast with the nematic thermotropic phases, Haller's equation cannot be used to predict the evolution of the quadrupolar splittings (Deltanu(Q) values), and hence the order parameters, versus T in the PBLG mesophases. For these particular lyotropic systems, it is shown that an exponential function of the form Deltanu(Q)[Hz]=C x exp (-E/RT[K]) provides excellent agreement between the experimental and expected Deltanu(Q) values. Analysis of the results reported in this work suggests that orientation and chiral discrimination phenomena in these lyotropic solvents could be treated separately because they would involve different interaction mechanisms.

Pulse shaping effect on two-photon excitation efficiency of α-perylene crystals and perylene in chloroform solution

We demonstrated that the two-photon excitation efficiency of perylene in chloroform solution as well as that of crystalline perylene was dramatically increased by optimizing the shape of intense femtosecond laser pulses of a regenerative amplifier output. The efficiency was three times higher than for an unshaped single femtosecond pulse with the pulse width of shorter than 50 fs. The pulse shape optimized for the solution sample was a pulse train with a repetition frequency of about 340 cm(-1), and the pulse shape optimized for crystalline perylene was very similar. These results supported our previous findings on alpha-perylene crystals using weak femtosecond pulses from a mode-locked laser oscillator [T. Okada et al. J. Chem. Phys. 121, 6385 (2004)]. Furthermore, it was confirmed that the shaped pulse optimized for the liquid sample could also increase the two-photon excitation efficiency of alpha-perylene crystals and vice versa. We concluded that the mechanism for the increase in excitation efficiency of perylene in chloroform was almost the same as that for alpha-perylene crystal, and that the efficiency increased mainly through intramolecular dynamical processes. Processes involving intermolecular interactions and/or energy states delocalized over the crystal cannot play the major role.

NMR study on the coordination of dibenzylideneacetone to chiral palladium(0) units. Fluxional behaviour including an intramolecular double bond exchange†

New derivatives with the general formula Pd(L–L)(η2-dba) (dba = dibenzylideneacetone), L–L = 1-diphenylphosphino-2,1′-(1-dimethylaminopropanediyl)ferrocene, PAPF, 1; 2-[1-(dimethylamino)ethyl]-1-(diphenylphosphino)ferrocene, PPFA, 2; N,N-dimethyl-1-[1′,2-bis(diphenylphosphino)ferrocenyl]ethylamine, BPPFA, 3; 1-diphenylphosphino-2,1′-(1-dicyclohexylphosphinopropanediyl)ferrocene, PPCyPF, 4 were synthesized from Pd2(dba)3·CHCl3 and the appropriate ferrocenyl ligand. When an excess of PPFA was used, the complex Pd(PPFA)3 with P-coordinated PPFA ligands was formed. The dba unit preferentially adopts an s-cis,trans conformation with the s-trans alkene coordinated to the Pd(L–L) fragment. Several intramolecular dynamic processes were identified: at room temperature a fast rotation of dba about the alkene–Pd bond, a Pd–N bond rupture process for complex 2 and, at elevated temperatures, an intramolecular diastereomer interconversion involving an alkene face exchange accompanied by an interchange between the coordinated and non-coordinated dba double bonds. The crystal structure of complex 2 was determined by X-ray diffraction.

Rhenium−Platinum Mixed-Metal Spiked-Triangle Clusters. Synthesis and X-ray Characterization of the Cluster Anions [Re2Pt(μ-H)2(CO)9{Re(CO)5}]-and [Re2Pt(μ-H)2(CO)9{HRe2(CO)9}]-

The spiked-triangle cluster [Re2Pt(μ-H)2(CO)9{HRe(CO)5}] (1), which contains a HRe(CO)5 molecule bound to the Pt vertex of a Re2Pt triangle, reacts with methanolic NEt4OH to give the red anion [Re2Pt(μ-H)2(CO)9{Re(CO)5}]- (2), whose structure has been determined by single-crystal X-ray analysis of its NEt4+ salt. NMR monitoring showed the intermediate formation of a carbomethoxy derivative 3, arising from the attack of OMe- on a carbonyl of the HRe(CO)5 group. The deprotonation of 1 can be performed also by a base as weak as pyridine. The anion 2 in solution is thermally unstable, and at room temperature it gives rise to complex mixtures. The main component has been identified, by X-ray analysis of its NEt4+ salt, as the anion [Re2Pt(μ-H)2(CO)9{HRe2(CO)9}]- (4), containing a HRe2(CO)9- complex bound to the Pt vertex of a Re2Pt triangle, through a Re(μ-H)Pt bond. The anions 2 and 4 are better obtained by treating [Re2Pt(μ-H)2(CO)8(COD)] (COD = 1,5-cyclooctadiene) with [Re(CO)5]- or [HRe2(CO)9]-, respectively, in the presence of CO. Experiments have been performed to clarify the origin of the [HRe2(CO)9]- fragment bound to Pt in 4. The reaction between free [HRe(CO)5] and [Re(CO)5]- produces in high yields the anion [HRe2(CO)9]-, but this one does not displace Re(CO)5- from 2. The reaction between [Re(CO)5]- and the complex 1 gives instantaneously the anion 2. No reaction occurs upon treatment of the anion 2 with [HRe(CO)5]. Variable-temperature NMR has revealed the occurrence in both the anions 3 and 4 of an intramolecular dynamic process equalizing the two hydrides bound to the Pt atom, with Ea = 70(1) and 74(7) kJ/mol, respectively.

High resolution photoelectron spectroscopy and femtosecond intramolecular dynamics of H2CO+ and D2CO+

High resolution helium Iα (584 Å) photoelectron spectra of H2CO and D2CO are reported. The present study reveals much new vibrational structure detail in the ionic first excited state of formaldehyde. Weak excitations of the ν3 (in H2CO) and ν1 (in D2CO) modes along with the strong excitations of the ν2 mode in the ionic first excited states are fully resolved for the first time. The weak excitations of the ν4 out-of-plane bending mode in the ionic ground and first excited states of formaldehyde cations indicate that they may have nonplanar equilibrium geometries. Strong isotope effects on vibronic (vibrational) couplings are observed in the cation first and second excited states. Vibrational autocorrelation functions are calculated from the high-resolution photoelectron spectra. The correlation functions calculated for the first electronic excited states show rather slow decay rate on the femtosecond time scale. The ultrafast decay of the formaldehyde cations implied by the correlation functions calculated for the third electronic excited states suggest that dissociation and intramolecular dynamic processes are the main decay pathways.

Stereochemistry of diorganotin(IV) bis(8-quinolinolate) and bis(8-quinolinethiolate) complexes in solution studied by NOE-difference spectroscopy

It was proved by 1H NOE-difference spectroscopy that all diorganotin(IV) complexes of type R 2Sn(oxin) 2 and R 2Sn(txin) 2 (where oxin = 8-quinolinolate, txin = 8-quinolinethiolate, R = butyl, phenyl, benzyl and vinyl, respectively) in chloroform solution have a distored cis-octahedral coordination at the metal centre. Two oxygen (sulphur) atoms occupy positions trans to each other, while the estimated bond angles OSnO (SSnS) are in the range 150–165°. Nitrogen atoms are cis to each other and the bond angles NSnN are less than 90°. Intramolecular dynamic processes have to be taken into account.

Bimetallic complexes containing lithium and boron. Studies on the reactions of (ArBMe 3)Li · OEt 2 complexes with the electron donors OEt 2 and THF by means of 1H, 7Li, 11B, and 13C NMR spectroscopy

The stabilities of the toluene and THF solutions of complexes (Ar 1BMe 3)Li·OEt 2 (I), Ar 1 = C 6H 4CH 2NMe 2−2 and (Ar 2BMe 3)Li·OEt 2 (II) and Ar 2 = C 6H 4-( S)-CHMeNMe 2−2 were investigated at various temperatures by means of 1H, 7Li, 11B, and 13C NMR spectroscopy. I is stable in toluene and only inversion of configuration on nitrogen and on C(1) aromatic carbon was observed. II is less stable in toluene than I, and beside the intramolecular dynamic processes dissociation with release of ether was detected. Complex I reacts with THF with donation of ether to THF and product IV, Ar 1BMe 3)Li·THF, was isolated and characterized in toluene solution. IV reacts further with THF forming V, [LiMe] n ·[THF], with cleavage of the LiC 2B bond. II dissolved in THF leads to the (Ar 2BMe 3)(Li· nTHF) complex, VI. The mechanism of the bond cleavage of the LiC 2B core in I and in II in the reaction with THF is discussed.

Preparation of Single Rotation Vibration States of CH3O (X̃2E) Above the H – CH2O Dissociation Threshold by Stimulated Emission Pumping

AbstractBy the technique of Stimulated Emission Pumping (SEP), single high‐lying rotation vibration states of CH3O (X̃2E) with defined total energy Err and total angular momentum J were probed in the regime 3000 cm−1 ≤ Err ≤ 9300 cm−1, up to well above the H – CH2O asymptotic dissociation limit (≈ 6800 cm−1). Observed low resolution SEP spectra exhibit distinctive vibrational band structure. Spectra recorded at high resolution reflect intramolecular dynamical processes occuring on different time scales. The detailed spectroscopic characterization of the level structure of highly excited CH3O provides the basis for critical tests of models for unimolecular reactions.

Dynamic Stereochemistry of Phosphaalkenes

Abstract The degenerative Z,E-isomerization at the P=C bond in phos-phaalkenes I and II has been studied by dynamic NMR spectroscopy. The complete bandshape analysis was used to evaluate the activation parameters for intramolecular dynamic process.

Synthesis, structure and properties of stereochemically non-rigid molybdenum pyrazolylborato complexes containing a dihapto-thiocarboxamido ligand

Pyrazolylboratomolybdenum complexes containing the η 2-CSNMe 2 ligand have been prepared by treating the appropriate carbonylmetallate anion with Me 2NCSCl. The structure of pzB(pz) 3Mo(CO) 2(η 2-CSNMe 2) (IIIb) has been established by X-ray crystallographic methods. The 1H and 13C NMR spectra of IIIb show evidence for two separate intramolecular dynamic processes in solution. Complex IIIb can be alkylated at the sulphur atom and forms 1/1 complexes with mercuric halides.

Metallocene bis(aza-crown ether) ligands and related compounds. Their syntheses, co-ordination chemistry, and electrochemical properties

The syntheses of new ferrocene and ruthenocene bis(aza-crown ethers)(11)–(14) containing amide bond linkages between the metallocene redox centre and aza-15-crown-5 (1,4,7,10-tetraoxa-13-azacyclopentadecane) and aza-18-crown-6 (1,4,7,10,13-pentaoxa-16-azacyclo-octadecane) are described. Variable-temperature 13C n.m.r. studies on these compounds reveal a common intramolecular dynamic process involving rotation about the N–CO bond whose energy barrier to rotation is cation (Na+ or Li+) dependent. The sodium cation forms 1 : 2 stoicheiometric complexes with the ferrocene bis(aza-crown ethers), whereas potassium produces a 1 : 1 intramolecular sandwich complex with the former and a 1 : 2 complex with the latter. Electrochemical investigations reveal that binding of Na+ and K+ guest cations at the respective aza-crown ether co-ordinating sites results in shifts of the ferrocene oxidation wave to more positive potentials. Lithium produces analogous electrochemical effects with these two compounds, however 13C n.m.r. results suggest this cation exclusively co-ordinates to the respective amide carbonyl oxygen donor atoms. This rationalisation is supported by ‘model’ ferrocene bis(tertiary amide) derivatives which exhibit redox-responsive behaviour to Li+ while being electrochemically insensitive to Na+ and K+.

Kinetics of the reaction between oxygen atoms and ethyl radicals

The kinetics of the O + C2H5 reaction have been investigated experimentally in a heatable tubular reactor coupled to a photoionization mass spectrometer. The reactants were generated homogeneously in the reactor by the simultaneous in situ photolysis of SO2 and diethyl ketone. The rate constant was determined from time-resolved measurements of C2H5 decay profiles under conditions of [O] in express [O]/[C2H5]0 > 20. The value of the O + C2H5 rate constant [(2.2 ± 0.4)×10–10 cm3 molecule–1 s–1] was independent of temperature (295–600K) and density [(3–12)×1016 molecule cm–3]. The chemical branching was also characterized both experimentally and theoretically. In the experimental study, the absolute yields of products from three reactive routes were determined and found also to be independent of temperature (298–450K) and density (same range). The branching fractions are 0.32 ± 0.07 for CH2O + CH3, 0.40 ± 0.04 for CH3CHO + H and 0.23 ± 0.8 for C2H4+ OH. In the theoretical study, the master equation describing the many possible intramolecular dynamical processes of the excited C2H5O formed in the initial step of this reaction was solved. RRKM theory was used to obtain the transition probabilities for each step. Potential–energy surface information needed to obtain the transition probabilities was obtained using BAC–MP4 electronic structure calculations. Theoretical branching fractions are in good agreement with those measured for the two major routes. Far less C2H4+ OH is predicted than observed, suggesting that these products are primarily produced by another process, viz. direct metathesis.