Double Minimum Potential Research Articles

1H NMR studies of proton transfer equilibria in hydrogen bonds – the role of the entropy

The influence of the entropy on the proton transfer equilibria AH···B [Formula: see text]A–···H+B in the dimethylphosphinic acid (DMP)-N-base and in the methanesulfonic acid (MSA)-N-oxide hydrogen bonds is studied by 1H NMR spectroscopy as a function of the ΔpKa, i.e., the pKa of the base minus the pKa of the acid. It is shown that with increasing ΔpKa the proton transfers to the N-oxide acceptor. In the DMP + N-base family of systems, first the double-minimum proton potentials (ΔH = 0) occurs and only with further increasing ΔpKa do the POH···N [Formula: see text] PO–···H+N equilibria become symmetrical (ΔG = 0). ΔG and ΔH are connected by the Gibbs–Helmholtz equation, ΔG = ΔH –TΔS. Hence, the reason of this effect is the large negative interaction entropy term (ΔSI) arising from the large order around the polar structure, which shifts the equilibria strongly to the left-hand side. With the MSA-N-oxide hydrogen bonds a single-minimum proton potential was found. The minimum shifts with increasing ΔpKa from the donor to the acceptor of the O···H+···N hydrogen bonds. It is shown that with increasing ΔpKa the proton potential becomes on average symmetrical, whereas the proton is still largely on the left-hand side. This result is also explained by the large negative interaction entropy due to the order of the environment if the proton is at the acceptor B.Key words: entropy, hydrogen bonds, infrared, spectroscopy, proton transfer.

X-Ray and spectroscopic re-investigation of urotropine–p-nitrophenol complex

The product of the reaction between urotropine and p-nitrophenol, reported as a 1:1 adduct that belongs to the triclinic P1 space group, is, in fact, a 1:2 [C6H12N4]·[p-HOC6H4NO2]2·[H2O] hydrate that crystallizes in the monoclinic C2 space group. In the crystal structure, one nitrogen atom of the urotropine moiety is linked by a hydrogen bond to the hydroxyl group of a p-nitrophenol molecule [N⋯O=2.655(7)Å]; the [C6H12N4]·[p-HOC6H4NO2] entities are linked into a linear one-dimensional chain through the lattice water molecule [O⋯N=2.871(8),O⋯O=2.878(7)Å]. The other p-nitrophenol moiety is disordered over two positions across the two-fold axis; its interaction with adjacent water molecules [O⋯O=2.53(1), 2.57(2)Å] holds neighboring chains together. The FTIR spectrum of the complex in the solid indicate the formation of OH⋯N intermolecular hydrogen bond described by an asymmetric double minimum potential and weak Zundel's polarizability. The spectroscopic measurements in solution demonstrate the dissociation of the complex observed in the solid and prove the presence of the 1:1 complexes urotropine–p-nitrophenol. In these complexes no proton transfer from p-nitrophenol to urotropine is observed.

Hole-burning spectra of tropolone–(CO 2) n ( n=1,2) van der Waals complexes and density functional study

The hole-burning, fluorescence excitation, and dispersed fluorescence spectra of jet-cooled tropolone (TRN)–(CO 2) n ( n=1,2) complexes are measured to investigate the structures of the complexes and the effects of intermolecular interaction on proton tunneling in TRN. The electronic transitions of TRN–(CO 2) n ( n=1,2) are well separated in the hole-burning spectrum. Only the transitions of one species have been identified for the 1:1 and 1:2 complexes. Structures of TRN–(CO 2) n ( n=1,2) are optimized by the density functional theory calculations at the B3LYP/cc-pVDZ level. Three local minima have been obtained for both the 1:1 and 1:2 complexes. In the 1:1 complex CO 2 is bonded in the molecular plane close to a solvation site, CO (Isomer I), CO⋯H–O (Isomer II), or CO (Isomer III). These complexes are stabilized mainly by the dipole–quadrupole interaction, and the binding energies for these complexes are estimated to be much smaller than those for the hydrogen-bonded complexes. Isomers I and II are plausible candidates for the observed species. The calculations imply that asymmetry of the double-minimum potential well along the tunneling coordinates of TRN–(CO 2) 1 is large enough to quench proton tunneling. This prediction is consistent with the nonobservation of the tunneling splittings even for the excitation of the tunneling promoting mode ν 13(a 1) or ν 14(a 1) of TRN.

Symmetry of N-H-N hydrogen bonds in 1,8-bis(dimethylamino)naphthalene.H+ and 2,7-dimethoxy-1,8-bis(dimethylamino)naphthalene.H+.

In solution, are the hydrogen bonds in monoprotonated N,N,N',N'-tetramethyl-1,8-naphthalenediamines single- or double-well? To answer this question, isotopic perturbation of equilibrium is applied to a mixture of -d(0), -d(3), -d(6), -d(9), and -d(12) isotopologs. The N-methyls of the 2,7-dimethoxy analogue show intrinsic isotope shifts from the geminal CD(3) and from only one distant CD(3), an unusual stereochemical effect transmitted across the hydrogen bond. The (13)C NMR splittings and intensities at the various ring carbons of both ions are consistent with perturbation isotope shifts, intrinsic shifts, or a combination of both. The perturbation shifts mean that the protons reside in a double-minimum potential and that each ion is a pair of rapidly interconverting tautomers. The significance of this result for the role of low-barrier hydrogen bonds in enzyme-catalyzed reactions is discussed.

Intermolecular double proton transfer controlled by laser field

We simulate a simultaneous double proton transfer controlled by laser fields in carboxylic dimers. The motions of two protons are described by the double minimum potential. Our strategy is to move the population efficiently from the one well to the other by using laser field. In order to incorporate the relaxation effect, the time-evolution of the system is numerically investigated by the density matrix theory. From the resultant solution, we discuss the effective pulse sequence for the control of double proton transfer.

On the molecular and vibrational structure of 1,6,6aλ4-trithiapentalenes. Analysis of the “bell-clapper’' asymmetrical S–S–S stretching mode

The molecular and vibrational structure of the 1,6,6aλ4-trithiapentalene (TTP) ring system was studied by experimental and theoretical procedures. IR absorption spectra were recorded of 2,5-dimethyl-1,6,6aλ4-trithiapentalene (DMTTP) in liquid solution, in a stretched polyethylene matrix, and in solid state tablet samples. The linear dichroism observed in the stretched polymer sample provided experimental symmetry assignments of the observed vibrational states. The results of B3LYP and B3PW91 density functional theoretical calculations were in good agreement with the observed molecular geometries and vibrational transitions for TTP and DMTTP. The computed molecular structures were characterized by sulfonium ylide-like Mulliken charge distributions (positively charged, three-coordinated sulfur center in position 6a, negatively charged carbons in positions 2, 3a and 5), consistent with the large dipole moments reported for these species. Of particular interest was the strong vibrational transition observed around 187 cm−1 in the far-IR spectrum of DMTTP, similar to the transition previously observed at 153 cm−1 for TTP. These transitions must be assigned to the asymmetrical S–S–S stretching vibration, the so-called “bell-clapper” mode. According to B3LYP and B3PW91 calculations the potential is U-shaped, corresponding to a negative anharmonicity constant xe in the order of − 0.025. Anharmonic effects are predicted to increase the frequency of the fundamental transition by about 5%. Hartree–Fock (HF) theory predicts a double-minimum potential for this mode, while post-HF Moller–Plesset second-order perturbation theory (MP2) predicts a single-minimum potential with a complicated shape and a positive anharmonicity.

Vibrational spectra of the adduct of 1,8-bis(dimethylamino)naphthalene with dichloromaleic acid (DMAN · DCM)

The infra-red (IR), Raman (R) and inelastic incoherent neutron scattering (IINS) spectra, particularly in low frequency region, of the title ionic adduct were studied. It is shown that all low frequency vibrations (below 200 cm −1) of (CH 3) 2N groups of protonated 1,8-bis(dimethylamino)naphthalene (DMAN) — clearly observed in IINS spectra — are sensitive to the environment, i.e. to the type of counterion forming short contacts with CH bonds of methyl groups. The internal frequencies were also calculated by ab initio method. The results are consistent with numerous observations of the counteranion effect on the geometry of the protonated DMAN. The conclusions are compared with structural and NMR studies reported recently for the 1,8-bis(dimethylamino)naphthalene with dichloromaleic acid (DMAN · DCM) adduct. The single crystal R polarized spectra taken over the frequency range 20–3200 cm −1 were analyzed in detail. We have shown that a substantial difference in the IR spectrum of the dichloromaleic acid (DCM) anion in the DMAN adduct and in the potassium salt results from different geometries of OHO hydrogen bonds. In the case of potassium salt the chains of longer intermolecular hydrogen bonds are formed described by means of a double minimum potential.

OHO]− and +[NHO]− hydrogen bonds in the 2:1 adduct of pentachlorophenol with 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene [(PCP)2·MTBD

The results of X-ray diffraction and IR spectroscopic studies for 2:1 pentachlorophenol-7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene [(PCP)2·MTBD] adduct are reported. The geometry of MTBD cations reflects the equal distribution of the positive charge among three nitrogen atoms. Short asymmetric [OHO]− hydrogen bonds with O⋯O distance of 2.508(2)Å and NH+⋯O− hydrogen bonds with N⋯O distance of 2.802(2)Å are formed showing broad IR absorption with two maxima located at ∼1200 and ∼2400cm−1. The second maximum is interpreted as due to the 0→2 transition between split levels in an asymmetric double minimum potential. One of the oxygen atoms forms an additional O⋯H–N+ hydrogen bonding with an MTBD cation. The situation is somewhat different in acetonitrile solution whose IR spectrum shows continuous absorption extended over whole the IR region. In acetonitrile, dissociation to free OHO− and +NH ions takes place and the OHO− bridges become dynamically symmetric. The broadening is interpreted in terms of a stochastic distribution of the geometry and the Zundel polarizability theory.

Construction of potential curves for diatomic molecular states by the IPA method

A program package is presented which utilizes the Inverted Perturbation Approach (IPA) method for construction of potential energy curves of 1Σ and 1Π states of diatomic molecules directly from the experimental data. In most cases using the constructed IPA potential one can reproduce the experimental energy levels within the accuracy of measurement. The package is successfully tested in case of double minimum potential where the traditional RKR method is not applicable.

Structural information on the S and S1 state of o-fluorophenol by hole burning and high resolution ultraviolet spectroscopy

The electronic transitions of o-fluorophenol situated at 36 799.382 cm−1 and 36 906.710 cm−1, denoted the A and B bands, respectively, have been investigated by high resolution fluorescence excitation spectroscopy. Hole burning studies together with the high resolution spectroscopy results show that both bands originate in the same ground state and can be fitted to the rotational constants of the cis isomer. The rotational constants for the excited states are found to be A′=3231.795 MHz, B′=2207.92 MHz and C′=1313.97 MHz for the A band and A′=3226.945 MHz, B′=2211.24 MHz and C′=1321.03 MHz for the B band. The planarity of the ground state is lost upon electronic excitation, which enhances the activity of an out-of-plane vibration. The A and B band transitions arise from excitations to respectively the zero and first overtone levels in the double-minimum potential of this out-of-plane vibration, which shows similarities to the so-called butterfly mode observed in other benzene derivatives.

Effects of a solvent molecule on the torsional potential of 9,9′-bianthryl

The laser-induced fluorescence and hole-burning spectra of 9,9′-bianthryl (BA) and its 1:1 complexes with rare gas atoms, N2, and water were obtained under a free jet condition. The torsional potentials of the BA complexes in the S1 state were determined from the observed progressions of the torsional vibration with a fitting program routine including a cross-correlation estimation. In the BA–He complex, the double-minimum torsional potential which is symmetric to the perpendicular conformation of the two anthryl moieties became slightly asymmetric. In contrast, the attachment of large rare gas atoms, N2 or H2O to BA significantly affected the torsional motion due to its local steric repulsion, and resulted in the reduction or loss of one wing of the potential valleys of the double minimum torsional potential.

Proton spin-lattice relaxation and local symmetry of the H bond inRb3H(SO4)2

A rather strong and unusual temperature and frequency dependence of the proton spin-lattice relaxation time ${T}_{1}$ has been observed in a ${\mathrm{Rb}}_{3}{\mathrm{H}(\mathrm{S}\mathrm{O}}_{4}{)}_{2}$ single crystal in the temperature interval between 300 and 10 K. Below 10 K the proton ${T}_{1}$ becomes temperature independent but still exhibits a rather strong Larmor frequency dependence. At higher temperatures the spin-lattice relaxation is determined by a modulation of the Rb-proton and/or proton-proton dipolar coupling due to thermally activated jumping of the proton between the two equilibrium sites in the H bond whereas at low temperatures phonon-assisted proton incoherent tunneling takes over. The results thus show that the double-minimum potential of the H bond in ${\mathrm{Rb}}_{3}{\mathrm{H}(\mathrm{S}\mathrm{O}}_{4}{)}_{2}$ is locally asymmetric in the temperature range investigated. There is no evidence for a change from a double to a single minimum type H-bond potential down to 4 K.

Ultrafast Proton-Transfer and Coherent Wavepacket Motion of Electronically Excited 1,8-Dihydroxyanthraquinone in Liquid Benzyl Alcohol Solution

Ultrafast pump-probe experiments with a time-resolution of 30 fs have been carried out to explore the non-radiative relaxation dynamics of electronically excited 1,8-dihydroxyanthraquinone (DHAQ) in polar liquid solution. The results are discussed in terms of a Lippincott-Schroeder double-minimum potential along the proton-transfer reaction coordinate for the ground (S

Proton tunneling of tropolone in durene single crystal as studied by time-resolved EPR detected excitation spectroscopy

We have measured the excitation spectra for tropolone–OH in durene single crystal and tropolone–OD in deuterated durene using a time-resolved electron paramagnetic resonance (TREPR) detection method that makes possible to separate the signals due to magnetically different sites. The tunneling doublet with 3 cm−1 was observed in the sharp zero-phonon line. The small splitting indicates that the crystal field increases the barrier of double-minimum potential for the proton tunneling in the S1 state. Moderately asymmetric potentials of the S0 and S1 states, where the energetic imbalance between two wells in the S1 state potential is opposite the S0 state potential, reasonably explained the observed unusual intensity ratio of the tunneling doublet (01+<01−). A well-resolved progression of a phonon band with a 15 cm−1 separation was also obtained in durene crystal at very low temperature. From a Franck–Condon analysis of the relative intensity of the phonon band, it was clarified that the stable configuration of the excited state tropolone in durene differed from that of the ground state.

Conformational properties of bis(difluoromethyl) ether as studied by microwave, infrared, Raman spectroscopy and by ab initio computations

Bis(difluoromethyl) ether, CHF2–O–CHF2 (E134), was studied by microwave, infrared and Raman spectroscopy and by quantum chemical methods. Ab initio calculations show the existence of three conformations with H–C–O–C–H arranged antiperiplanar–synperiplanar (AS), antiperiplanar–antiperiplanar (AA) and synclinal–synclinal (gauche–gauche, GG), with relative enthalpies of ca. 0, 8 and 4kJmol−1, respectively. Infrared spectra of the vapour and of the amorphous solid and crystalline state were obtained between 4000 and 10cm−1. The spectra of the compound isolated in argon and nitrogen matrices at 5K were also recorded. Raman spectra of the cooled liquid and of the amorphous and crystalline solids were obtained in the region 4000–20cm−1. Two conformers with an estimated enthalpy difference of 7±2kJmol−1 were detected in the liquid phase. The matrix isolation spectra also suggest the existence of more than one conformer in the vapour phase. The vibrational spectra were assigned with the aid of normal coordinate calculations employing harmonic force fields from the ab initio study. The microwave spectrum was investigated in the 10.0–38.0GHz spectral region at 195K. One conformer, antiperiplanar–synperiplanar with one H–C–O–C dihedral angle of 177° and the other 12° from syn, was assigned. The ground state spectrum is accompanied by a series of strong lines from the vibrational excited states; of these, five states of the ν21 torsional mode were assigned and analysed in terms of a double minimum potential with a barrier of 19cm−1 in the syn position.

Ab initio molecular orbital study of the isomerization reaction surfaces of C 3 and C 3−

The potential-energy surfaces of C 3 and C 3 − in the ground and some low-lying excited states are investigated by the CASSCF and MRD-CI methods to clarify the features of the isomerizations between the linear and bent structures. In addition, the asymmetric linear structures corresponding to the antisymmetric stretching mode in the A ̃ 1 Π u state are examined. It is shown that the a ̃ 3 Π u state of C 3 is isomerized into the equilateral triangle structure via an asymmetric transition state. Also, the central barrier height (215 cm −1) calculated for the double-minimum potential of A ̃ 1 Π u agrees reasonably well with the experimental data (284.3 cm −1).

Molecular and Supramolecular Structures ofN-Phenyl Formamide and its Hydrated Clusters

The amide, N-phenyl formamide (formanilide), and its water clusters have been studied in a jet expansion using laser-induced fluorescence excitation and mass-selected, resonant two-photon ionization (R2PI) techniques. The isomer with a trans configuration of the amide group (defined in Figure 1) is identified through analysis of the partially resolved contour of its S1 ← S0 band origin. Ion-dip “hole-burn” spectra of the nonplanar cis isomer contain either symmetric or antisymmetric components of low-frequency progressions, providing evidence of a double-minimum ground state potential. Excited-state vibrations at 76 and 152 cm-1, which are strongly Franck−Condon active, show evidence of Duschinski mixing of the ground-state modes including Cring−N torsion. Water clusters have been observed for trans-formanilide only: two distinct 1:1 hydrates, two 1:2 hydrates, and a complex with at least four bound water molecules. (The cis isomer is also expected to form extremely stable complexes with water, but none have been detected experimentally in the present study.) The observed clusters are assigned using spectroscopic data, including band contours, to structural alternatives computed ab initio at the HF/6-31G* level. The 1:1 hydrates are assigned to a cluster in which water binds at the NH site and one in which water binds at the HCO site. In the 1:2 clusters, the addition of a further water molecule to each of the 1:1 clusters results in cyclic hydrogen-bonded structures, with the water dimer bridging between proton donor and proton acceptor sites of the host. The interactions are HCO···HOH and OCH···OH2 in one case and NH···OH2 and πring···HOH in the other. At the MP2/6-31G*//HF/6-31G* level, these structures are ca. 10 kJ mol-1 more stable than the nearest competitor, in part because of cooperative effects. The R2PI spectrum of the NH bound 1:2 cluster “C” is very similar to that of the indole(H2O)2 complex assigned by Zwier and co-workers.19 Its origin is red-shifted 482 cm-1 from trans-formanilide, and the electronic transition excites long intermingled vibrational progressions with frequencies of 29, 38, and 51 cm-1.

Computational studies on acyclic amidyl radicals: Π and Σ states and conformations

Restricted open and unrestricted Becke3LYP/6-31+G(d) calculations on Π and Σ states as well as equilibrium geometries of the formamidyl radical (1) and four of its dialkyl substituted derivatives 2–5 have been carried out. While all radicals studied are significantly twisted about the RN–C(O) bond and show a Π-type total spin density, the calculations confirm the special status of N-tert-butyl acetamidyl (4) that was found with EPR spectroscopy. Each of the torsional double-minimum potentials of N-methyl and N-isopropyl radicals 2, 3, and 5 shows a low barrier to interconversion for two equivalent conformers whereas 4 is situated in a steeper well with a larger twist angle which explains reported EPR 13C hyperfine splittings.

Deuterium NMR Study of Unstable Phenomena and Water Molecular Dynamics in Samarium Nitrate Hexahydrate Crystal

Deuterium NMR spectra, spin−lattice relaxation time T1, and differential thermal analysis (DTA) were measured for Sm(NO3)3·6D2O. The magnetization recovery can be divided into two components. From deuterium NMR spectra and T1, the short and the long components of T1 were found to be mainly dominated by the 180° flip of the crystallization and the coordinated water molecules, respectively. For the nonannealed sample, the reproducibility of T1 was observed. The influence of the instability was not seen in the long component of T1. The results of the long component of T1 gave the activation energy Ea = 20 kJ mol-1 and the correlation time at infinite temperature τc0 = 7.8 × 10-12 s for the 180° flip of the coordinated water molecule. The motion of the crystallization water at low temperatures can be explained by the 180° flip in the asymmetric double minimum potential. τc0 = 1.2 × 10-12 s, Ea = 16 kJ mol-1, and the energy difference in the potential wells ΔE = 2.0 kJ mol-1 were obtained for the 180° flip of ...

Investigation of the potential energy surfaces for the ground [formula omitted] and excited [formula omitted] electronic states of SO 2

The stationary points along the dissociation path are investigated by means of high-level ab initio methods and the reliability of different methods is discussed. The multi-reference AQCC method using the ANO-type basis set is shown to give geometrical parameters and relative energies in very good agreement with experiment. At this level of theory, the C ̃ 1 B 2( 1 A′) state has an asymmetrical equilibrium geometry and double-minimum potential with a barrier of 170 cm −1, in good agreement with experimental data.