NMR studies of elementary steps of hydrogen transfer in condensed phases

Limbach, Hans−Heinrich; Scherer, Gerd; Meschede, Ludger; Aguilar‐Parrilla, Francisco; Wehrle, Bernd; Braun, J.; Hoelger, Ch.; Benedict, Hans; Buntkowsky, Gerd; Fehlhammer, Wolf Peter; Elguero, José; Smith, J. A. S.; Chaudret, Bruno

doi:10.1063/1.45383

Cited by 10 publications

(25 citation statements)

References 2 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Unfortunately, the precision of X-ray measurements does not allow to obtain equilibrium constants of tautomeric processes by analysis of experimental effective bond lengths and thermal ellipsoids. 15 NH4Cl and the appropriate benzoyl chloride according to the method of Axenrod. 21 The crystals for XRD analysis were grown from anhydrous tetrahydrofuran or ethanolic solutions.…”

Section: Theoretical Sectionmentioning

confidence: 99%

Combined X-ray Diffraction and ¹⁵N CPMAS NMR Study of Molecular Structure and Proton Order/Disorder Phenomena in Cyclic N,N‘-Bisarylformamidine Dimers

et al. 1997

View full text Add to dashboard Cite

Crystal structures of a series of five symmetrically substituted N,N-bisarylformamidines ArNHsCHdNAr with Ar ) X-C 6 H 4 , X ) p-OCH 3 (IV), p-CH 3 (V), p-F (VI), p-NO 2 (VII), and m-Br (VIII) have been determined by single-crystal X-ray diffraction (XRD) and complete the series studied previously where X ) H (I), p-Br (II), and p-Cl (III). In addition, the results of variable-temperature 15 N CPMAS NMR experiments performed on 15 Nlabeled I, II, and IV are reported. All compounds form cyclic dimers linked by two NsH‚‚‚N hydrogen bonds which can form two different tautomers, a and b, interconverting by fast double proton transfers. The NMR experiments indicate three types of amidines characterized by different magnitudes of the equilibrium constants K ab of the tautomerism. In dimers of type such as V-VIII, we find K ab , 1 (i.e., only a single tautomer in the temperature range between 100 and 300 K). In this case, the hydrogen-bonded protons are ordered and can be localized by XRD. Furthermore, the C‚‚‚N bond lengths and torsional and valence angles involving the two aryl groups of an amidine unit are different. For dimers such as II and III, characteristic temperature dependent 15 N CPMAS NMR line shape changes are observed indicating that K ab ) 1 within the margin of error. Rate constants of the tautomerism can in this case be obtained by line shape analysis. For this degeneracy to occur, the aryl group conformations at both amidine nitrogen atoms must be similar. XRD then observes disordered hydrogen-bond protons and, in principle, also disordered nitrogen atoms. However, in practice, the disorder of the latter is not resolved leading to the observation of equalized C‚‚‚N bond lengths. Finally, dimers (I, IV) represent an intermediate case with K ab < 1, which could be labeled as "dynamic partial order". The implications of the molecular structure and the hydrogen bond and proton transfer characteristics are discussed.

show abstract

Section: Theoretical Sectionmentioning

confidence: 99%

Combined X-ray Diffraction and ¹⁵N CPMAS NMR Study of Molecular Structure and Proton Order/Disorder Phenomena in Cyclic N,N‘-Bisarylformamidine Dimers

et al. 1997

View full text Add to dashboard Cite

show abstract

“…The banier of rotation of coordinated dihydrogen is for example very sensitive to the back-bonding from the metal to dihydrogen [2, 31. Similarly, some transition metal polyhydrides display very large H-H couplings J in their NMR spectra due to a pairwise coherent quantum mechanical exchange [4]. In addition, they also exhibit incoherent exchange processes characterized by a rate constant k leading to 'H NMR line broadening and coalescence [5]. In several papers it has been postulated that the two phenomena are related to each other [6]; moreover, the J and k appear to be "twins"; if they are of the order of Hz to KHz they influence the 'H Nh4R spectra, and if they are of the order GHz to THz they appear in the Inelastic Neutron Scattering (INS) spectra [6c].…”

Section: Introductionmentioning

confidence: 99%

Coherent and incoherent dihydrogen dynamics in a ruthenium trihydride complex with the tris(pyrollyl)phosphine ligand

Gründemann

Limbach

Rodrı́guez

et al. 1998

Ber Bunsenges Phys Chem

View full text Add to dashboard Cite

The ruthenium IV trihydride complex Cp*RuH3(Ppy3) (1) accommodating the π‐accepting tris(pyrollyl)phosphine ligand has been synthesized by standard methods. This complex exhibits average exchange couplings J of the coherent dihydrogen exchange and average rate constants k of the incoherent dihydrogen exchange which are significantly larger than the corresponding triphenylphosphine and tricyclohexylphosphine analogs. No kinetic HH/HD isotope effects on the incoherent exchange are observed within the margin of error. Using a simple kinetic model involving incoherent vibrational excitation it is shown that in principle both the coherent and the incoherent processes should be related. However, whereas the energy of activation of k corresponds to the barrier of rotation, the energy of activation of the coherent dihydrogen exchange J is much smaller i.e. determined by states well below the barrier. Nevertheless, it is found that chemical modification which reduces the energy of activation of J also reduces the energy of activation of k which demonstrates the relation between both kinds of processes.

show abstract

“…J exch adds up with the usual scalar magnetic coupling J magn to an effective J coupling, as was recognized by Zilm et al 5a-d and Weitekamp et al5e J exch represents an average over a large number of rovibrational states as it increases strongly with increasing temperature. On the other hand, superimposed on the coherent exchange are incoherent exchange processes, which also have been observed in the NMR spectra of these hydrides ,, . In the case of a HD pair, these incoherent processes correspond to H/D scrambling between the two different molecular sites in which the pair is located.…”

Section: Introductionmentioning

confidence: 75%

“…This splitting can be observed by inelastic neutron scattering 2 (INS) when the barrier is small and the splittings are on the order of terahertz. In the other extreme, when the barrier is large and the tunnel splitting becomes of the order of typical 1 H chemical shift differences (i.e., hertz to kilohertz) the tunnel splitting gives rise to a quantum exchange coupling X 12 = J exch in the NMR spectra , of the hydride pairs, which can be observed if each hydride exhibits a different chemical shift. J exch adds up with the usual scalar magnetic coupling J magn to an effective J coupling, as was recognized by Zilm et al 5a-d and Weitekamp et al5e J exch represents an average over a large number of rovibrational states as it increases strongly with increasing temperature.…”

Section: Introductionmentioning

confidence: 99%

²H NMR Theory of Transition Metal Dihydrides: Coherent and Incoherent Quantum Dynamics

et al. 1997

View full text Add to dashboard Cite

In this paper a simple phenomenological description of the effects of coherent quantum and incoherent mutual exchange of two deuteron nuclei in solid state transition metal complexes on their 2 H NMR spectra is given. This description is based on the quantum-mechanical density matrix formalism developed by Alexander and Binsch. Only the nuclear spin system is treated quantum mechanically. The quantum exchange interaction in NMR is included in the nuclear spin Hamiltonian, and the interaction with the surrounding bath and incoherent exchange processes are treated as phenomenological rate processes described by rate constants. The incoherent exchange corresponds formally to 180°rotations or jumps of the D-D vector around an axis perpendicular to this vector and averages the different quadrupole splitting of the two deuterons. In principle the dideuteron pair will exist in several rovibrational states. However, if the interconversion among these states is fast, the dideuteron exchange can be described by an average exchange coupling or tunnel frequency X 12 and a single average rate constant k 12 of the incoherent exchange. It is shown that the incoherent exchange gives rise to a relaxation of rate -2k 12 between coherences created between states of different symmetry. The 2 H NMR line shape of a dideuteron pair in the solid state as a function of tunnel and incoherent exchange rate is studied numerically. For single crystals, the effects of coherent and incoherent exchange are strongly different, in particular if the rate constants are on the order of the quadrupole splitting. The spectra of nonoriented powder samples are more similar to each other. Nevertheless, our calculations show that there are still pronounced differences, which should allow the distinction between coherent and incoherent exchange even in nonoriented samples.

show abstract

NMR studies of elementary steps of hydrogen transfer in condensed phases

Cited by 10 publications

References 2 publications

Combined X-ray Diffraction and ¹⁵N CPMAS NMR Study of Molecular Structure and Proton Order/Disorder Phenomena in Cyclic N,N‘-Bisarylformamidine Dimers

Combined X-ray Diffraction and ¹⁵N CPMAS NMR Study of Molecular Structure and Proton Order/Disorder Phenomena in Cyclic N,N‘-Bisarylformamidine Dimers

Coherent and incoherent dihydrogen dynamics in a ruthenium trihydride complex with the tris(pyrollyl)phosphine ligand

²H NMR Theory of Transition Metal Dihydrides: Coherent and Incoherent Quantum Dynamics

Contact Info

Product

Resources

About

NMR studies of elementary steps of hydrogen transfer in condensed phases

Cited by 10 publications

References 2 publications

Combined X-ray Diffraction and 15N CPMAS NMR Study of Molecular Structure and Proton Order/Disorder Phenomena in Cyclic N,N‘-Bisarylformamidine Dimers

Combined X-ray Diffraction and 15N CPMAS NMR Study of Molecular Structure and Proton Order/Disorder Phenomena in Cyclic N,N‘-Bisarylformamidine Dimers

Coherent and incoherent dihydrogen dynamics in a ruthenium trihydride complex with the tris(pyrollyl)phosphine ligand

2H NMR Theory of Transition Metal Dihydrides: Coherent and Incoherent Quantum Dynamics

Contact Info

Product

Resources

About

Combined X-ray Diffraction and ¹⁵N CPMAS NMR Study of Molecular Structure and Proton Order/Disorder Phenomena in Cyclic N,N‘-Bisarylformamidine Dimers

Combined X-ray Diffraction and ¹⁵N CPMAS NMR Study of Molecular Structure and Proton Order/Disorder Phenomena in Cyclic N,N‘-Bisarylformamidine Dimers

²H NMR Theory of Transition Metal Dihydrides: Coherent and Incoherent Quantum Dynamics