Application of Nuclear Magnetic Relaxation To Elucidate Proton

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Application of Nuclear Magnetic Relaxation to Elucidate Proton Location and the Dynamics of N…H…O type Hydrogen Bond Masuda Yuichi, and Tomoko Nakano J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp303297c • Publication Date (Web): 01 Aug 2012 Downloaded from http://pubs.acs.org on August 9, 2012

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The Journal of Physical Chemistry

Application of nuclear magnetic relaxation to elucidate proton location and dynamics in N···H···O hydrogen bonds

Tomoko Nakano, Yuichi Masuda* Department of Chemistry, Faculty of Science, Ochanomizu University, Bunkyo-ku, Tokyo 112-8610, Japan E-mail: [email protected]

Received:

TITLE RUNNING HEAD: Intramolecular proton transfer rates in solution CORRESPONDING AUTHOR FOOTNOTE: Department of Chemistry, Faculty of Science, Ochanomizu University, Bunkyo-ku, Tokyo 112-8610, Japan E-mail: [email protected] Phone & fax: +81-3-5978-5350

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ABSTRACT: The proton location and dynamics in a hydrogen bond in solution are fundamentally important for understanding the phenomenon of proton transfer (PT). In the present study, the proton location and its dynamics were explored for the NH form of the two PT tautomers of the Schiff base by analyzing the fluctuation of the 15N-1H magnetic dipolar coupling by the PT as well as the NH reorientational motion. For this purpose, the 15N and 13C spin-lattice relaxation times were measured in dichloromethane or acetonitrile solutions of three Schiff bases with different substituents on the benzene moieties, N-(4,6dimethoxysalicylidene)-methylamine (compound 1), N-(1-methylnitrilomethylidyne)-2-naphthalenomethylamine (compound 2), and N-(3,5-dibromosalicylidene)-methylamine (compound 3). For the NH form of compound 2 in dichloromethane, the proton location shifted to the center between the nitrogen and oxygen atoms, as compared with the minimum of the PT potential surface derived from molecular orbital calculations. For the NH form of compound 3 in dichloromethane, the proton location shift was not observed, and the PT rate was significantly lower than the reorientation rate of the NH bond. The results are discussed in terms of the electronic effect of the substituents and the static and dynamic solvent effect.

KEYWORDS: intramolecular proton transfer, hydrogen bond, solution, NMR, spin-lattice relaxation, Schiff base

BRIEFS: The proton location and dynamics in the intramolecular N···H···O hydrogen bonds of three Schiff bases in solutions have been explored through measurements of the spin-lattice relaxations ascribed to the magnetic dipolar interaction between the 1H and 15N nuclei.


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1. Introduction

Intramolecular proton transfer (PT) along hydrogen bonds has played an important role in numerous chemical and biological reaction systems as an elementary process of acid-base reactions.1-5 The potential surface for PT is a central issue in understanding the PT process, and, in most cases, the experimental information on this issue has been provided as a hydrogen-bond structure derived by means of single-crystal structural analyses from X-ray and neutron scattering.4 Among the structural information, the hydrogen locations in the hydrogen bonds are particularly important for judging the shapes of the PT potential surfaces, e.g., whether they have single or double wells.6-10 On the other hand, some of these hydrogen-bond structural studies have indicated that the hydrogen location is significantly affected by intermolecular interactions or the crystal packing.9-11 For example, the NH distances in the N···H···N intramolecular hydrogen bond of 1,8bis(dimethylamino)naphthalene cation (DMANH+) in the crystalline state depend on the counter anion and the crystalline form,11 which indicates the obvious effect of the intermolecular interaction on the hydrogen location. In solution, the solvent, especially a polar solvent, should also affect the hydrogen location or the PT potential surface. However, the effect of the environment on the hydrogen location and dynamics in solution is different from that in the crystalline state because of the fluctuations of the intermolecular interaction in time. The PT potential surface also accordingly fluctuates because of the rotational and translational motions of the solvent molecules.2,12-14 The solvent thus contributes to the PT potential surface and, as a result, to the PT rate in the following two ways: First, an additional reorganization energy is produced by a change in the equilibrated solvent orientation or in the configuration between the reactant and the product. This reorganization energy increases the reaction barrier and, as a consequence, decreases the rate within the regime of the transition state theory (TST).1216

Second, the rotational and translational dynamics of the solvent plays a critical role in the PT,

particularly when the intrinsic barrier is comparable with or even lower than the solvent reorganization energy. The PT rate is then controlled by the dynamics of the solvent.2,17-19 ACS Paragon Plus Environment

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Understanding the solvent’s contribution to the PT rate in a static and dynamic manner is thus a central issue in ultrafast, i.e., low-barrier, PTs, which are often found along hydrogen bonds. Various theoretical studies of this topic, including molecular dynamics simulations, have been conducted.2,19-24 Recent progress in hybrid quantum mechanics/molecular dynamics (QM/MD) methods has explicitly paved the way for investigating the effect of a solvent on the electronic structure as well as the quantum mechanical behavior of vibrations.25-30 On the other hand, in experimental studies the hydrogen location and dynamics in hydrogen bonds in solution is often presumed from the H/D isotope effects on NMR chemical shifts9,31-34 and the deuteron quadrupole coupling constants (eQq).35 However, more direct information on the hydrogen location and dynamics is quite limited, except for a few studies that directly address the issue using analysis of the contribution of the magnetic dipolar interaction to the spin-lattice relaxation times for N···H···N and O···H···O hydrogen-bond systems.36-38 In the present study, we attempt to evaluate the hydrogen (proton) location and its dynamics, i.e., the PT rate between the NH and the OH form, in Schiff bases through the 15N spin-lattice relaxation time, T1, attributed to the 15N-1H magnetic dipolar interaction (see Scheme 1). The Schiff bases, which serve as model systems possessing an N···H···O hydrogen bond, are particularly important in evaluating the solvent contribution, because with the adoption of proper substituents on the benzene moieties, close energies between the respective NH and OH forms can be achieved. Then, the PT tautomerism equilibria are almost completely controlled by the solvent.39-41 A considerably low PT barrier is also important for the participation of solvent fluctuations in the PT dynamics.2,12-14,17-19 We have selected three Schiff bases, as shown in Scheme 2. In these Schiff bases, the NH and OH forms coexist in considerable amounts in solution.33,39,40,42-44 This indicates that the energy difference between the tautomers is within ~10 kJ/mol. In addition, the differences in the substituents on the benzene moieties provide electronically different effects on the N···H···O hydrogen bonds. For example, the electrondonating character of the methoxy groups of compound 1 induces an increase in the nitrogen basicity, in contrast with the bromo groups of compound 3, which have an electron-withdrawing character.10,45 In addition, the naphthalene group in compound 2 enlarges the size of the conjugated π system.46 One ACS Paragon Plus Environment

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purpose of the present study is to illustrate how the difference in the electronic effect of the substituents changes the hydrogen (proton) location and the dynamics in the N···H···O hydrogen-bond systems. The effects of the solvent and the substituent on the tautomeric equilibria for various Schiff bases have been intensively studied by use of NMR methods, e.g., using the 15N-1H spin-spin coupling constants,39 the primary and the secondary deuterium isotope effect on the 13C and 15N chemical shifts,47-52 and the UV spectra.2,40 In these studies, the shifts in the tautomeric equilibria are discussed in terms of the electronic and steric effects of the substituents and the solvent polarities. Thus, we have significant knowledge about the factors that change the energies of the NH and the OH forms. On the other hand, the experimental approach for investigating the hydrogen-bond structures or the location of the hydrogen atoms in the hydrogen bonds of the NH and OH forms in solution is rather limited; nevertheless, the information thus obtained is directly connected to the PT potential surface. One of the few examples of an experimental approach to addressing this issue is found in the studies performed by the Limbach group.48,49 In these studies, the contributions of the zero-point energies to the deuterium isotope effects on the 15N shifts are analyzed by combing various NMR measurements and considering a distinction between the intrinsic and equilibrium contributions to the H/D isotope effects on the NMR shifts in O···H···N hydrogen-bonded systems. Here, we use 15N spin-lattice relaxation attributed to the 15N-1H magnetic dipolar interaction as a direct tool for observation of the proton (hydrogen) location and dynamics, i.e. the PT rate. The 15N-1H magnetic dipolar interaction in the solid state has often been applied to analyze the structures of hydrogen-bond systems including nitrogen atom(s) because of the inverse cubic dependence of the magnitude of the local magnetic field by the nuclear spins, such as protons, near the 15N nucleus.53-56 In solution, on the other hand, the dynamics of the molecules, e.g., the reorientational motion, causes the 15 1

N-1H dipolar interaction to fluctuate, so that the 15N spin-lattice relaxation time attributed to the 15N -

H magnetic dipolar interaction, T1dd(NH), depends on the correlation time of the reorientation of the

NH vector.57,58 The T1dd(NH) value is also dependent on the PT dynamics, since the PT, i.e., the proton jump, causes the NH distance to fluctuate. The contribution of the PT dynamics to the spin-lattice ACS Paragon Plus Environment

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relaxation induced by the magnetic dipolar interaction in solution has already been discussed in detail.36,37 In the present paper, we attempt to determine the proton location and its dynamics, i.e., the PT rates, for the chosen Schiff bases in dichloromethane and acetonitrile solutions from the 15N spinlattice relaxation times, taking into account the NH reorientational motions.

2. Experimental

2.1. Materials

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N-(4,6-dimethoxysalicylidene)-methylamine (1), 15N-(1-

methylnitrilomethylidyne)-2-naphthaleno-methylamine (2), and 15N-(3,5-dibromosalicylidene)methylamine (3), were synthesized using a method described in the literature.50 Methanol solutions of 98% 15N-enriched methylamine hydrochloride (Cambridge Isotope Laboratories, Inc.) and the equivalent amount of sodium hydroxide were added to methanol solutions containing the respective aldehydes: 4,6dimethoxysalicyl-aldehyde (Sigma-Aldrich, Inc.), 2-hydroxy-1-naphthalaldehyde (Wako Pure Chemical Industries, Ltd.), and 3,5-dibromosalicyl-aldehyde (Acros Organics). The solutions were then refluxed. The products were recrystallized from carbon tetrachloride or chloroform. In order to obtain a deuterated sample of each, a hundred-fold equivalence of D2O (99.96 atom% D (Merck)) was added to tetrahydrofuran solutions of the corresponding 15N-enriched Schiff bases. The solutions were stirred for one hour, and then the solvents were evaporated.

2.2. NMR Measurement

The 13C and 15N NMR spectra were obtained with a JEOL

LA-400 Fourier transform spectrometer (100.40 MHz for 13C, 40.40 MHz for 15N) at 9.4 T. The NOESY measurements were carried out with a Bruker AVANCE Fourier transform spectrometer (600.13 MHz for 1H) at 14.1 T. The spin-lattice relaxation times, the T1 values, were measured by an ACS Paragon Plus Environment

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inversion-recovery method with a pulse sequence of π-pulse–t–π/2-pulse, t being the pulse interval time. The 13C NMR spectra were obtained under 1H irradiation. The JNH values were determined from the 15N NMR measurements without 1H irradiation. 1H irradiation was not applied for the15N T1 measurements, because of the smaller signal intensity with 1H irradiation than without for compound 3 due to the nuclear Overhauser enhancement (NOE) greater than −1. 59 The FIDs were accumulated 160–260 times in the 13C measurements and 28–40 times in the15N measurements for each t, and 15–20 different t values were used. The maximum t values were set at ca. 10 times the respective T1 values. The factors of the 13C NOE factors were obtained from the signal intensities under complete 1H decoupling and gated decoupling. The spectral intensities were determined by fitting the spectrum with a Lorenzian-toGaussian function. NOESY measurements were carried out for compound 1 in order to determine the conformation of the methoxy groups in solution. For all the NMR measurements, dichloromethane-d2 (99.9 atom%D [Cambridge Isotope Laboratories, Inc.]) or acetonitrile-d3 (99.96 atom%D [Sigma-Aldrich, Inc.]) was used as the solvent. The measured sample solutions were prepared by vacuum distillation after the solvents were dried and degassed. The tubes were then torch-sealed. These procedures were carried out with a vacuum line. The solution concentration was adjusted in the range 0.10–0.12 M. The measurements were performed with 5 or 10 mm (outer diameter) cylindrical tubes. The temperature was controlled to within ±0.5 K with a JEOL GVT2 temperature control unit. The temperature elevation of the sample solutions due to the 1H irradiation in the 13C NMR measurements was minimized by making the 1H irradiation power as low as possible. The sample temperatures were calibrated with a thermistor set in a dummy sample under the corresponding NMR measurement conditions, such as under 1H irradiation. In the 15N NMR measurements on the solutions of the deuterated samples, signals from NH species that originated from the entrainment of small amounts of H2O during the preparation of the NMR samples were observed (see Figure 1). On the other hand, the 15N T1 values for the NH and ND signals in each deuterated sample solution were the same within the experimental error, because the rates of 1H or 2D exchange with water were faster than T1 (20–60 s). The observed 15N T1 values can thus be ACS Paragon Plus Environment

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regarded as the average values of those corresponding to the NH and ND species. The 15N T1 values for the ND species were determined from the T1 values observed in the solutions of the undeuterated and the corresponding deuterated samples and the ratio of the NH and ND 15N signal intensities in the deuterated sample solutions (see Table S1). The T1 values and the NOE factors were measured several times or more, and their reproducibility was confirmed to be within several percent for the measured T1 values and within 0.1 for the values of the NOE factors.

2.3. Molecular orbital calculation

Ab initio molecular orbital (MO) calculations for

the equilibrium structures and the dipole moments of the chosen Schiff bases were carried out using DFT/B3LYP with the base function 6-311++G(d,p) in the program packages Gaussian 09 or Spartan 10. The equilibrium structures considering the solvent (dichloromethane or acetonitrile) were calculated using the DFT method at the B3LYP/6-311+G(d,p) level with the SCI-PCM solvation model.

2.4. Determination of conformation of methoxy groups

There could be two possible

conformers for the methoxy groups of compound 1, since the results of the MO calculations (DFT/B3LYP, 6311++G(d,p) level) indicate that the energy of conformer 1(b), which has the lowest energy, is only 10 kJ/mol lower than that of the second most stable conformer 1(a) (see Scheme 3). The conformation of the methoxy groups was then confirmed by NOESY measurements in dichloromethane. Only the cross peaks between 5C-H and -OCH3 were observed in the NOESY spectra. Conformer 1(a) is therefore dominant for both the NH and OH forms in solution.

3. Results and Discussion 3.1. Basic principle

Here, we briefly describe the principle used to obtain the NH

distance and the PT rate from the 15N T1 value attributed to the 15N-1H magnetic dipolar interaction, T1dd(NH). The value of T1dd(NH) depends on the magnitude of the local magnetic field, Hloc, produced ACS Paragon Plus Environment

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by the proton, which is inversely proportional to the cube of the NH distance, and the correlation time of the fluctuation of Hloc. In solution, this fluctuation is, in most cases, produced by the reorientational motion about the NH vector, which changes the angle between the static magnetic field and the NH vector. On the other hand, when the system involves a PT process, the proton jump is accompanied by a change in the distance between the nitrogen and hydrogen atoms, and it produces a field fluctuation that contributes to the longitudinal relaxation of the 15N magnetization just like the NH reorientation. More general discussions and the formulation for the spin-lattice relaxation time in connection with PT in 15

N-1H…15N hydrogen bond in detail are given by Kowalewski et al.36 When PT time, τPT, and the NH reorientational correlation time have a comparable timescale,

T1dd(NH) depends on both the correlation time of the NH reorientation and the PT time, as can be seen in eq. 1a.60

2

(

) (

)

2 2 µ  T1dd ( NH)−1 =  0  γ H2 γ N2 h 2  PNH POH rNH −3 − rN..H −3 τ '+ PNH rNH −3 + POH rN..H −3 τ R(NH)     4π 

τ '-1 = τ PT-1 + τ R(NH)-1 (1a). Where γΗ and γΝ are the magnetogyric ratios of 1H and 15N respectively, and µ0 and ħ denote their usual meanings. PNH and POH denote the population fractions for the NH and the OH form, respectively, rNH and rN···H represent the NH distance in the NH form and the NH hydrogen-bond distance in the OH form, respectively. Here we use a common correlation time of the NH reorientation, τR(NH), for the NH and the OH form for each Schiff base because the difference in the values is negligible small as described in section 3.3. In the slow PT limit, i.e., when τPT >> τR(NH), the observed T1dd(NH)−1 does not depend on τPT and is represented as an average of the T1−1 values for the respective NH and OH forms, as shown in eq. 1b:


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µ  −6 6 T1dd ( NH)−1 =  0  γ H2 γ N2 h 2 ( PNH rNH τ R(NH) + POH rN−L Hτ R(NH) ) 4 π  

(1b)

In the fast PT limit, i.e., when τPT τR(NH), respectively. On the other hand, the geometrically optimized structure in the MO calculations (DFT/B3LYP/6-311++G(d,p)) for compound 1 gave 1.04 Å as the NH bond distance in the NH form. The results of the calculations including the solvent model (SCI-PCM) gave similar distances. (Table 5) NH distances of 1.03–1.05 Å have also been reported based on DFT/B3LYP calculations with the same basis set for Schiff bases with various substituents on the benzene moiety, including compound 1, in the absence of serious steric constrain.72 Thus the rNH value at the fast PT limit seems to correspond to that obtained in the MO calculations, considering the error in estimation of rNH, ±0.01 Å (see Figure 2). On ACS Paragon Plus Environment

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the other hand, an effect of potential anharmonicity conduces to an inflation of the bond distance obtained from the magnetic dipolar interactions (vibrational averaging effect),73 and the inflation is increased up to ~3% in hydrogen-bonds with typical double-well potentials.74 If the inflation in the NH bond distance by the vibrational averaging effect is taken into account, the span of the τPT value of compound 1 in dichloromethane is presumed to be similar to or lower than τR(NH) (see Figure 2). In the acetonitrile solution, the experimentally obtained rNH values assuming various τPT were somewhat longer than those obtained in dichloromethane although the difference was comparable level of the estimated inaccuracy in rNH (see Figure 2). Considering the shorter NH reorientational correlation time in acetonitrile than that in dichloromethane, the result indicates that the PT rate in acetonitrile is probably somewhat higher than that in dichloromethane because no significant difference between the NH bond distances in these two solvents is found in the MO calculations with SCI-PCM solvent model (see Table 5). In the case of compound 2 in dichloromethane, the rNH values expected from T1dd(NH) and T1dd(CH) under both the slow and fast PT limits were significantly longer than those obtained in the MO calculations (see Table 5). The discrepancy in the distances is probably ascribed to substantial degree of the anhamonicity in the PT potential surface in the NH-form site. In the case of hydrogen-bond with typical double-well potential, the effect of the anharmonicity is presumed to be less than several percent inflation in the bond distance.72 On the other hand, in low barrier hydrogen-bond system the proton distribution becomes broader and the asymmetry in the distribution increases (see Figure 3(a)). The observed T1dd(NH) -1 is connected to an average of the local magnetic field by the proton, which is distributed to a significant extent particularly toward the center between the nitrogen and the oxygen atom. Because the rNH value obtained from T1dd(NH) corresponds to the average of the inverse of the cube of the distances from the proton distributed asymmetrically. The longer rNH values for compound 2 in Table 5 than that obtained from the MO result, which corresponds to the potential minimum position, are thus attributable to the substantial anharmonic PT potential surface of the NH-form site (see Figure 3(a)). The considerable anharmonicity of the PT potential surface is compatible with the low PT barrier ACS Paragon Plus Environment

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potential as well as the proton delocalization. In this situation the PT rate is possibly much faster than the reorientational motion of the NH bond (τPT τR(NH) were significantly shorter than and similar to, respectively, that predicted from the MO calculations, even if one considers the considerable ambiguity in rNH (±0.02– 0.03 Å) from the larger error in the τR(NH) value on the analysis of the rotational anisotropy. These results indicate that the PT time, τPT, is, at least, significantly slower than the NH reorientation. Accordingly, the PT potential well for the NH form is significantly deep and is close to harmonic (see Figure 3(b)). The substitution on the benzene moiety thus varies the anharmonicity around the bottom of the PT potential surface that causes the shift of the average of the proton position from the potential minimum to the center between the nitrogen and the oxygen atom. The difference in the potential anharmonicity or in the proton distribution could be related to the energy difference between the NH and OH forms. This can be seen in the results of DFT calculations of the potential surfaces and the corresponding proton wave functions for some Schiff bases obtained by Filarowski et al.22 In these results, the proton distribution maximum for the NH form shifts from the adiabatic potential minimum to the center between the nitrogen and oxygen atoms as the destabilization of the NH form increases. In the present study, however, the trend of the shifts of the distribution do not correlate with the observed enthalpy differences between the NH and OH forms (see Table 1). Consequently, the results can probably be attributed to differences in the electronic properties of the substituents. For example, in compound 2, which exhibited prominent the shifts in the proton distribution, the expansion of the conjugated π system by the naphthalene ring in the salicylidene is expected to promote the proton delocalization in the N···H···O hydrogen bond by the “resonance assistance effect.”75-77 As a result, the anharmonicity around the bottom of the PT potential surface is increased. Such a higher extent of proton delocalization in compound 2 probably conduces the higher PT rate. On the other hand, for compound 3, less anharmonicity around the bottom of the PT potential surface for the NH form and the ACS Paragon Plus Environment

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relatively high PT barrier may be attributable to the weakening of the hydrogen bond by the withdrawing character of the bromo substituents.

3.6. Solvent contribution to the PT rate

A significant contribution of solvent to the

tautomerism equilibria of the chosen Schiff bases can be found in the experimental ∆H values and the energies obtained in the MO calculations for the tautomers, as can be seen in Table 1. Here, we consider the effect of the solvent on the PT rates based on a model including the dynamical solvent contribution presented by Hynes et al.17,18 For simplicity, the reaction is considered to be separated into the following two steps: (i) a process along the solvent coordinate suffering solvent dielectric friction to a point at which the free energies of the reactant and the product are equal and (ii) the intrinsic nonadiabatic tunneling PT process at this point on the solvent coordinate. The PT rate, kPT, is then represented in terms of the rates of these two processes, ks and kna, respectively, as follows:17,18

kPT−1 = kna−1 + 2ks−1

(6)

The rate of the process along the solvent coordinate, ks, depends on the solvent reorganization energy, λs, and the longitudinal solvent dielectric relaxation time, τL,17,18 i.e.,

1/ 2

βλ s    16π 

 k s = τ L−1 

 − βλ s  exp    4 

(7)

β = (k BT )

−1

By regarding the solvents as dielectric continua, the λs values for each compound in dichloromethane and acetonitrile were estimated from the solvent parameters given in literature78,79 and the molecular radius and the dipole moment change between the reactant and the product obtained in the MO calculations, as shown in Table 6. ACS Paragon Plus Environment

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The estimated ks value for compound 3 in dichloromethane exceeds the PT rate, ~ τPT–1, which is presumed to be significantly lower than the inverse of the NH reorientational correlation time, τR(NH)−1 ~ 1011 s−1, as described in section 3.5. The PT rate in compound 3 is therefore considerably determined by kna, i.e., the rate of the intrinsic nonadiabatic tunneling PT process. On the other hand, the proposed PT rates for compound 1 in dichloromethane and acetonitlile are similar to or higher than the estimated ks values. The result indicates that the PTs are, at least partially, controlled by solvent dynamics. The slight acceleration of the PT rate for compound 1 in acetonitrile compared with that in dichloromethane, as described in section 3.5, may be attributed to the higher ks value owing to the much shorter longitudinal dielectric relaxation time, τL, of acetonitrile. In other words, the faster solvent fluctuation of acetonitrile plays a role in the acceleration of the PT rate. In the case of compound 2, the contribution of the ks term to the total PT rate, kPT, is suspected to be major because the kPT value is presumed to be higher than those for compound 1 and 2.

4. Conclusion The proton location and dynamics in the hydrogen bonds of the three Schiff bases in dichloromethane or acetonitrile were explored by the measurement of the 15N and 13C spin-lattice relaxation times through analysis of the 15N-1H magnetic dipolar interactions and the fluctuations. For compound 3, which has a substituent of electron-withdrawing character, the PT rate is sufficiently slower than the reorientation of the NH vector. When the substituents are changed to methoxy groups with electron-donor properties (compound 1), the PT rate seems to increase to a rate similar to or greater than the inverse of the reorientational correlation time, τR(NH)−1 ~ 1011 s−1. In contrast, for compound 2, which has an expanded conjugated π system, it was found that significant proton distribution shift from the minimum position of the PT potential surface for the NH form to the center of the hydrogen bond associated with the potential anharmonicity. Accordingly, the results are connected to the low PT


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barrier and to the higher PT rate, possibly far beyond τR(NH)−1 ~ 1011 s−1. The variation in the PT rates was considered based on a model including the contribution of solvent fluctuations presented by Hynes et al.17,18 For compound 1 and 2, with its higher PT rates, the solvent dynamics, at least partially, contributes to the PT rate. The present method is thus one of the few methods to deduce the proton location and dynamics (PT rate) in solution experimentally, which is important for comprehensively evaluating the solvent contribution to PT and the PT potential surface

ACKNOWLEDGMENT This work was partially supported by a Grant-in-Aid for Scientific Research, No. 19550012, from the Ministry of Education, Science and Culture. The authors thank Dr. Y. Mori for valuable discussion and help with the molecular orbital calculations. The authors also thank Dr. Akiko Masuda for assistance to prepare the revised manuscript.

SUPPORTING INFORMATION 15

N spin-lattice relaxation times in the deuterated sample solution (Table S1). Plots of ln

(PNH/POH) against 1/T (Table S2 & Figure S1). Geometry data for compounds 1, 2, and 3 for the determination of the reorientational correlation times (Tables S3 and S4). This information is available free of charge via the Internet at http://pubs.acs.org.

REFERENCES and NOTES

(1) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: New York, 1997.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(2) Keifer, P. M.; Hynes, J. T. Ultrafast Hydrogen Bonding Dynamics and Proton Transfer Processes in the Condensed Phase; Elsaesser, T.; Bakker, H. J. Eds.; Kluwer Academic Publishers: Dordrecht, 2002.

(3) Paola, G.; Valeria, F.; Valerio, B.; Gastone, G. Adv. Mol. Struct. Res. 1996, 2, 67-102.

(4) Jeffery, G. A. Crystallogr. Rev. 2003, 9, 135-176.

(5) Marx, D. Chemphyschem 2006, 7, 1848-1870.

(6) Yaghmaei, S.; Khodagholian, S.; Kaiser, J. M.; Tham, F. S.; Mueller, T. L. J.; Morton, T. H. J. Am. Chem. Soc. 2008, 130, 7836-7838.

(7) Thomas, L. H.; Florence, A. J.; Wilson, C. C. New J. Chem. 2009, 33, 2486-2490.

(8) del Amo, J. M. L.; Langer, U.; Torres, V.; Buntkowsky, G.; Vieth, H.-M.; Pérez-Torralba, M.; Sanz, D.; Claramunt, R. M.; Elguero, J.; Limbach, H.-H. J. Am. Chem. Soc. 2008, 130, 8620-8632.

(9) Kong, S.; Lesnichin, S. B.; Hartl, M.; Daemen, L. L.; Eckert, J.; Antipin, M. Yu.; Shenderovich, I. G. J. Phys. Chem. A 2011, 115, 8041-8048.

(10) Chatziefthimiou, S. D.; Lazarou, Y. G.; Hadjoudis, T.; Mavridis, I. M. J. Phys. Chem. B 2006, 110, 23701-23709.

(11) Bienko, A. J.; Latajka, Z.; Sawka-Dobrowolska, W.; Ozeryanskii, V. A.; Pozharskii, A. F.; Grech, E.; Nowicka-Scheibe, J. Chem. Phys. 2003, 119, 4313-4319.

(12) Kuznetsov, A. M. Charge Transfer in Physics, Chemistry and Biology; Gordon and Breach: Luxembourg, 1995.

(13) German, E. D., Kuznetsov, A. M., Dogonadze, R. R. J. Chem. Soc., Faraday II 1980, 76, 11281146.

(14) Ghosh, S. K. J. Mol. Liq. 1993, 57, 75-90.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 38

(15) Marcus, R. A. J. Chem. Phys. 1965, 43, 679-701.

(16) Kurz, J. L.; Kurz, L. C. J. Am. Chem. Soc. 1972, 94, 4451-4461.

(17) Brogis, D. C.; Lee, S.; Hynes, J. T. Chem. Phys. Lett. 1989, 162, 19-26.

(18) Borgis, D.; Hynes, J. T. J. Chem. Phys. 1991, 94, 3619-3628.

(19) Cukier, R. I.; Morillo, M. J. Chem. Phys. 1989, 91, 857-863.

(20) Hanna, G.; Kapral, R. Acc. Chem. Res. 2006, 39, 21-27.

(21) Kiefer, P. M.; Hynes, J. T. J. Phys. Chem. A 2002, 106, 1834-1849.

(22) Filarowski, A.; Koll, A.; Hansen, P. E.; Kluba, M. J. Phys. Chem. A 2008, 112, 3478-3485.

(23) Eckert, M.; Zundel, G. J. Phys. Chem. 1988, 92, 7016-7023.

(24) Stare, J.; Panek, J.; Eckert, J.; Grdadolnik, J.; Mavri, J.; Hadži, D. J. Phys. Chem. A 2008, 112, 1576-1586.

(25) Borgis, G. Tarjus; H. Azzouz J. Chem. Phys. 1992, 97, 1390-1400.

(26) Bala, P.; Grochowski, P.; Lesyng, B. J.; McCammon, A. J. Phys. Chem. 1996, 100, 2535-2345.

(27) Tunõn, I.; Martins-Costa, M. T. C.; Millot, C.; Ruiz-López, M. F. J. Chem. Phys. 1997, 106, 36333642.

(28) Kim, S. Y.; Hammes-Schiffer, S. J. Chem. Phys. 2003, 119, 4389.

(29) Yamada, A.; Okazaki, S. J. Chem. Phys. 2006, 124, 094110.

(30) Yamada, A.; Okazaki, S. J. Chem. Phys. 2008, 128, 044507-044514.


22

Page 23 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(31) Altman, L. J.; Laungani, P.; Gunnarsson, G.; Wennerstrom, H.; Forsen, S. J. Am. Chem. Soc. 1978, 100, 8264-8265.

(32) Smirnov, S. N.; Golubev, N. S; Denisov, G. S.; Benedict, H.; Schah-Hohammedi, P.; Limbach, H.H. J. Am. Chem. Soc. 1996, 118, 4094-4101.

(33) Sharif, S.; Denisov, G. S.; Toney, M. D.; Limbach, H.-H. J. Am. Chem. Soc. 2006, 128, 3375-3387.

(34) Limbach, H.-H.; Pietrzak, M.; Sharif, S.; Tolstoy, P. M.; Shenderovich, I. G., Smirnov, S. N., Golubev, N. S.; Denisov, G. S. Chem. Eur. J. 2004, 10, 5195-5204.

(35) Jackman, L. M.; Trewella, J. C. R.; Haddon J. Am. Chem. Soc. 1980, 102, 2519-2525.

(36) Bernatowicz, P.; Kowalewski, J.; Sandstrom, D. J. Phys. Chem. A 2005, 109, 57-63.

(37) Wennerstrom, H. Mol. Phys. 1972, 24, 69-76.

(38) Masuda, Y.; Nakano, T.; Sugiyama, M. J. Phys. Chem. A 2012, 116, 4485-4494.

(39) Dziembowska, T.; Rozwadowski, Z.; Filarowski, A.; Hansen, P. E. Magn. Reson. Chem. 2001, 39, S67-S80.

(40) Antonov, L.; Fabian, W. M. F.; Nedeltchev,a D.; Kamounah, F. S. J. Chem. Soc., Perkin Trans. 2 2000, 1173-1179.

(41) Dobosz, R.; Skotnicka, A.; Rozwadowski, Z.; Dziembowska, T.; Gawinecki, R. J. Mol. Struct. 2010, 979, 194-199.

(42) Schilf, W.; Kamieński, B.; Dziembowska, T.; Rozwadoeski, Z.; Szady-Chełmíeniecka, A. J. Mol. Struct. 2000, 552, 33-37.

(43) Makal, A.; Schilf, W.; Kamieński, B.; Szady-Chełmíeniecka, A.; Grech, E.; Woźniak, K. Dalton Trans. 2011, 40, 421-430.


23


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Page 24 of 38

(44) Filarowski, A. J. Phys. Org. Chem. 2005, 18, 686-698.

(45) Rozwadowski, Z.; Majewski, E.; Dziembawska, T.; Hansen, P. E. J. Chem. Soc., Perkin Trans. 2 1999, 2809-2817.

(46) Dziembowska, T. Polish J. Chem. 1998, 72, 193-209.

(47) Dziembowska T.; Rozwadowski Z. Curr. Org. Chem. 2001, 5, 289-313.

(48) Benedict H.; Hoeger C.; Aguilar-Parrilla F.; Fehlhammer W.-P.; Wehlan M.; Janoshek R.; Limbach H. H. J. Mol. Struct. 1996, 378, 11-16.

(49) Smirnov S. N.; Golubev N. S.; Benedict H.; Schah-Mohammedi P.; Limbach H. H. J. Am. Chem. Soc. 1996, 118, 4094-4101.

(50) Hansen P. E.; Sitkowski J.; Stefaniak L.; Rozwadowski Z.; Dziembowska T. Ber. Bunsenges. Phys. Chem. 1998, 102, 410-413. Dziembowska, T.; Rozwadowski, Z.; Filarowski, A; Hansen, P. E. Magn. Reson. Chem. 2001; 39: S67–S80.

(51) Filarowski A.; Koll A.; Rospenk M.; Krol-Starzomska I. J. Phys. Chem. A 2005, 109, 4464-4473.

(52) Rospenk M.; Koll A.; Sobczyk L. Chem. Phys. Lett. 1996, 261, 283-288.

(53) Vener, M. V. Hydrogen-Transfer Reactions, Hynes, J. T. Ed.; John Wiley & Sons: New York 2007; Vol. 1; pp. 273-299.

(54) Ortlieb, M.; Havenith, M. J. Phys. Chem. A 2007, 111, 7355-7363.

(55) Wassermann, T. N.; Luckhaus, D.; Coussan, S.; Suhm, M. A. Phys. Chem. Chem. Phys. 2006, 8, 2344-2348.

(56) Neumann, M.; Brougham, D. F.; McGloin, C. J.; Johnson, M. R.; Horsewill, A. J.; Trommsdorff, H. P. J. Chem. Phys. 1998, 109, 7300-7311.


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(57) Abragam, A. The Principles of Nuclear Magnetism; Oxford University Press: New York, 1961; Chap. VIII.

(58) Farrar, T. C.; Becker, E. D. Pulse and Fourier Transform NMR; Academic Press: New York, 1971; Chap. 4. (59) No meaningful difference was observed for T1 for compound 3 with and without 1H irradiation. (60) Equation 13 in reference 36 for 15N-1H···15N hydrogen-bond systems is reduced to eq. 1a when an minor effect of the difference in the orientation between the local magnetic fields by the bonded and the hydrogen bonded proton is disregarded. Similarly, eqs 1b and 1c for the slow and fast PT limits are equivalent to eqs 14 and 15 in reference 36. In eq. 1a τPT−1 is related to the PT rate from the NH to the OH form, kNH, and from the OH to the NH form, kOH, as τPT−1 = kNH + kOH, and kNH/ kOH = POH/POH. (61) Lyerla, J. R.; Levy, G. C. Topics in Carbon-13 NMR Spectroscopy; Levy, G. C., Ed.; John-Wiley & Sons: New York, 1974; vol 2, Chap. 3.

(62) Jackman, L. M.; Trewella, J. C. J. Am. Chem. Soc. 1976, 98, 5712-5714.

(63) Kowalewski, J.; Effemey, M.; Jokisaari, J. J. Magn. Reson. 2002, 157, 171-177.

(64) Dill, K.; Allerhand, A. J. Am. Chem. Soc. 1979, 101, 4376-4378.

(65) Huntress, W. T. J. Chem. Phys. 1968, 48, 3524-3533.

(66) Vold, R. L.; Vold, R. R.; Canet, D. J. Chem. Phys. 1977, 66, 1202-1216. (67) The reason why this method is used to obtain the T1dd(NH) values instead of measurements of combinations of the 15N-T1 values and the NOE factors is its better accuracy for the determination of T1dd(NH) within a certain measurement time. In the present case, the measurement of the NOE factors to a sufficient accuracy takes a considerably longer time.

(68) Hoelger C. G.; Limbach, H. H. J. Phys. Chem, 1994, 98, 11803-11810.


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(69) Kikuta, Y.; Ishimoto, T.; Nagashima, U. Bull. Chem. Soc. Jpn. 2008, 81, 820-825., Udagawa, T.; Tachikawa, M. J. Mol. Strct. 2009, 912, 63-66.

(70) As can be seen in eqs. 1a and 1b under both the slow and fast PT limits, the contribution of the OH form to T1dd(NH)−1 is minor because of the much shorter NH distance in the NH form than in the OH form. In a typical case of rNH = 1.00–1.05 Å and rN···H = 1.70–1.80 Å of the NH and OH forms, respectively, of Schiff bases, the 15N···1H dipolar contributions to 15N-T1dd are estimated to be ca. 10% and 2% under the fast and slow PT limits, respectively, assuming PNH ~ POH .Thus, we used a fixed distance, 1.75 Å as the NH distances in the OH forms, rN···H, because the rN···H values for the same type of the Schiff bases with various substituents on the benzene moiety without serious steric constrains are distributed within a range, 1.7—1.8 Å in most cases.

(71) These values are based on the reproducibilities in the T1 measurements (within several percent) and those for the NOE factor (within 0.1). (See experimental section). An ambiguity in the τR(NH) values on the rotational anisotropy analyses (ca. 5%) and in the estimation in the PNH and POH from the arbitrary in JNH(NH form) (–90 ± 3 Hz) is also included. (72) Filarowski, A.; Glowiaka, T.; Koll, A. J. Mol. Struct. 1999, 484, 75-89.

(73) Henry, E. R.; Szabo A. J. Chem. Phys. 1985, 82, 4753-4761.

(74) Case, D. A. J. Biomol. NMR 1999, 15, 95-102.

(75) Gilli, G.; Bellucci, F.; Ferretti, V.; Bertolasi, V. J. Am. Chem. Soc. 1989, 111, 1023-1028.

(76) Alarcon, S. H.; Olivieri, A. C.; Cravero, R. M.; Gonzalez-Sierra, M. J. Chem. Soc. Perkin Trans. 2 1994, 5, 1067-1070.

(77) Alarcon, S. H.; Olivieri, A. C.; Labadie, G. R.; Cravero, R. M.; Gonzalez-Sierra, M. Tetrahedron 1995, 51, 4619-4626.

(78) Viswanath, D. S.; Natarajan, G. Data book on the Viscosity of Liquids; Hemisphere Publishing Corporation: Washington, 1989. ACS Paragon Plus Environment

26

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(79) Schrödle, S.; Annat, G.; MacFalne, D. R.; Forsyth, M.; Buchner, R.; Heffer, G. Chem. Commun. 2006, 1748-1750.


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Page 28 of 38

TABLES Table 1. 15N-1H spin-spin coupling constants, population fractions of NH forms, and thermodynamic parameters for PT equilibria

Compound 1 dichloromethane 1

JNH a) (Hz)

PNH a)

∆H c) (kJ/mol) ∆S c) (J/mol)

-49.8

acetonitrile -46.3

0.55 -13.2 42.5

∆E c),d) (kJ/mol) ∆E c),e) (kJ/mol)

2

dichloromethane dichloromethane -60.6

-13.7 b)

0.51

0.67

-7.10

-9.75

-7.11

23.4

26.7

38.1

11.3 -2.07

3

1.88 -4.24

-9.11

0.152

11.0 -4.44

a) Data at 298 K. b) Value extrapolated using those at lower temperatures (see text). c) Values relative to those for the OH forms. d) Values from MO calculations: DFT/B3LYP(6-311++G(d,p)). e) Values from MO calculations in solvent: DFT/B3LYP(6-311++G(d,p)) with SCI-PCM solvent model.


28

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2.

13

C spin-lattice relaxation times attributed to magnetic dipolar interaction, T1dd(CH),

and the parameters used to analyze the rotational anisotropies at 298 Ka) 13

C-T1 (s)

ηNOE

T1dd(CH) (s)

τR(CH) (10−11 s)

θ (o)

compound 1 in dichloromethane (θ0 = 56o, A = 1.28 ×10-11s, B = 1.44 ×10-11s) C3

2.96

1.70

3.47

1.42

59

C5

3.08

1.60

3.85

1.28

-59

C7

3.06

1.76

3.46

1.42

-128

compound 1 in acetonitrile o

(θ0 = 51 , A = 1.19 ×10-11s, B = 1.23 ×10-11s) C3

3.56

1.80

3.94

1.25

59

C5

3.66

1.75

4.17

1.18

-59

C7

3.72

1.83

4.03

1.22

-128

compound 2 in dichloromethane b) (θ0 = 120o, A = 1.19 ×10-11s, B = 1.45 ×10-11s) C4

3.15

1.71

3.67

1.34

-0.6

C7

3.41

1.63

4.17

1.18

-121

C9

3.06

1.77

3.44

1.43

-59

C10

3.50

1.73

4.03

1.22

-118

compound 3 in dichloromethane (θ0 = 137o, A = 0.90 ×10-11s, B = 2.11 ×10-11s) C4

2.92

1.74

3.33

1.48

-0.5

C6

3.78

1.52

4.95

0.99

-120

C7

4.03

1.71

4.69

1.05

-113

a) The number on each atom refers to Scheme 2. b) For compound 2 the 13C relaxation times and the NOE values were indicated for the 13C peaks without interference of overlap with the other peaks.


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Page 30 of 38

Table 3. Reorientational correlation times of NH vectors, τR(NH), at 298 K (10−11 s) a) Compound 1

2

3

dichloromethane acetonitrile

dichloromethane

dichloromethane

NH form

1.45

1.25

1.23

0.90

OH form

1.43

1.23

1.25

0.92

a) Angles of NH or N…H against vectors from C1 to C4: 59o (NH form) , 51 o (OH form) for compound 1; 8 o ( NH form) , 51 o (OH form) for compound 2; 46 o (NH form) , 42o ( OH form) for compound 3.

Table 4. 15N spin-lattice relaxation times at 298 K Compound 1 dichloromethane

acetonitrile

2

3

dichloromethane

dichloromethane

T1(NH) (s)

19.2

25.0

23.8

25.4

T1(ND) (s)

45.2

56.5

50.3

32.7

T1dd(NH) (s)

31.4

41.9

43.3


107

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Table 5. NH distances obtained from experiments and from molecular orbital calculations a) Compound 1

2

3

Experimentsb) 1.07

1.14

1.03

1.09 1.03

τPT>>τR in dichloromethane τPT>>τR in acetonitrile

1.10

0.94

1.04

τPT

Application of Nuclear Magnetic Relaxation To Elucidate Proton

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