Proton-proton Coupling Constant

Easily recognized splitting patterns found in various spectra provide the chemical shifts of the different sets of hydrogen that generate the signals differ by two or more ppm. The patterns are symmetrically distributed on both sides of the proton chemical shift, and the central lines are always stronger than the outer lines. The most commonly observed patterns have been given descriptive names, such as doublet (two equal intensity signals), triplet (three signals with an intensity ratio of 1:2:1) and quartet (a set of four signals with intensities of 1:3:3:1). The line separation is always constant within a given multiplet, and is called the coupling constant (J). The magnitude of J, usually given in units of Hz, is magnetic field independent. Coupling constants play an immense role in configurational and conformational studies. The relative position of protons is determining factor for Vicinal coupling constant between two protons. For example, in 1,2-disubstituted ethenes, the larger vicinal coupling constant was observed between the olefinic protons for the trans isomer 82a than for the cis isomer 82b [127,134].

The vicinal coupling constant depends on the dihedral angle between the protons in saturated systems. Karplus [118] gave equations 1 and 2 relating the coupling constant with dihedral angles.

J₁ = k₁cos²ï¦ – c (0 ï‚£  ï‚£ 90°) … (1)

J₂ = k₂cos²ï¦ – c (0 ï‚£  ï‚£ 180°)… (2)

These equations were later modified as equation 3.

J₂ = A cos²ï¦ – B cos²ï¦ + C … (3)

In equation 3, J is the coupling constant and A, B and C are constants related to the electro-negativities of the substituents attached to the C-C segment. The J value decreases markedly with increase in the electronegativities of the substituents [135-140].

13C-NMR:

Transitions of only 13C nuclei are noticed in 13C-NMR spectroscopy. Figure 3 represents different δ values (in ppm), couplings, coupling constants (in Hz) and chemical shifts of 13C nuclei processing in different chemical environments. Usually, δ value scale of 13C-NMR ranges from 0-220 ppm with respect to TMS as internal standard. 13C-NMR spectral interpretation can be best understood from chart given in figure 3 [126,127].

¹³C Chemical shift

As in the same ways of proton NMR spectrum, Chemical Shift in 13C NMR spectrum provides the hybridization (sp3, sp2, sp) of each carbon nucleus due to shielding and deshielding effects. Each carbon nucleus has its own electronic environment, different from the environment of other, non-equivalent nuclei. Figure 3: Chart representing 13C nuclei chemical shift due to different chemical environments. Electronegative atoms and pi bonds cause downfield shifts (“Thinkbook”). Spin-spin coupling provides the number of protons attached to the 13C nuclei. (i.e., primary, secondary tertiary or quaternary carbon) [126,127].

Carbon (¹³C) has a much broader chemical shift range. One important difference is that the aromatic and alkene regions overlap to a significant extent [126,127].Many factors such as inductive effects of substituent, hybridization state of the observed nucleus, Van der Wall’s and steric effects between closely spaced nuclei, electric fields originating from molecular dipoles or point charges, hyperconjugation, mesomeric interactions in  electron systems (delocalization effects), diamagnetic shielding due to heavy substituents (heavy-atom effect) and anisotropy effects is known to influence the ¹³C Chemical shift of six-membered ring compounds.

Among those factor, electrostatic effects due to the presence of a heteroatom in the cyclohexane moiety and steric perturbation effects being intrinsic importance. Lambert et al. [141] documented the effect of heteroatom in monoheterocyclohexanes 83 on the shifts of ring carbons. The -shift is a steep function of electronegativity of heteroatom X. A high frequency shift of about 50 ppm is produced by an increase in one unit electronegativity. However, a small effects of heteroatom electronegativity on  and -carbons are produced, a shift of -2.5 ppm/electronegativity unit for  and -5.0 ppm/electronegativity unit for -carbon, respectively. Ramalingam et al have demonstrated the effect of introduction of heteroatom in 84a-84e [86]. The decreasing order of the deshielding effect of heteroatom on the benzylic carbon is O > NMe > NH > S. because of a field effect, the heteroatom generates a low frequency an upfield shift in the carbonyl resonance.

Contrary to  and  effects, the -effect is being a property of at least four atoms and it has a torsional component. All γ – anti substituents cause increased shielding on C-5 due to the presence of α and γ protons. The γ – anti effect C-3 is found to be rather deshielding. The resonating carbon and perturbing  substituent showed the dihedral angle arrangement ranging from 0-180°. -gauche effects is found to be almost independent of the nature of the perturbing group X and generally occur in the 60-80° regions, whereas -anti effect in the 150-180° regions. The introduction of an axial substituent shifts the resonance of a -carbon to lower frequencies. The -anti effect (introduction of an equatorial substituent) is small. Interpretation of the substituent effects mainly depends on the steric and polar effects [142-144].

Based on the ¹³C NMR spectrum of vinylcyclohexane at low temperature, Buchanan observed the low frequency shifts in 85a relative to the equatorial counterpart 85b [145]. Based on the ¹³C NMR spectrum of various di-and tri-methylcyclohexanes, Dalling and Grant [146] observed an axial methyl group shifts the resonance of C(2), C(3) and C(4) at 1.40, 5.41 and 6.37 ppm and the corresponding resonance shifts for an equatorial methyl group at 5.96, 9.03 and 0.05 ppm, respectively. The shielding by an axial methyl group relative to an equatorial methyl group has been ascribed to steric interactions [142]. Furthermore, The 13C NMR data of 4 – hydroxypiperidines results indicate that substituent effects are markedly influenced by steric interaction. Eliel et al. [147] study on -effect of heteroatoms in heteracyclohexanes 86a-86d provide evidence that the -carbon located anti to a second-row heteroatom (X=O; NH) resonates at significantly lower frequency than the analogous carbon anti to a methylene group or a third-row heteroatom.

Pandiarajan et al. [13] suggested a method of assigning the configuration of a sub­stituent in saturated six–membered ring compounds, existing in chair conformation, from ¹³C chemical shift of a single epimer. Furthermore, the influence of the nearby substituents on the substituent parameters of equatorial methyl, gem-dimethyl, and equatorial and axial hydroxyl groups in several six-membered ring compounds 87a-87g has been suggested by Pandiarajan et al [13]. The magnitude of the  effect of a particular substituent is significantly reduced by a nearby substituent and the magnitude of the  effect decreases as the number of gauche interactions increases. Though, the  and  effects are not influenced by the nearby substituents [13].

Nuclear Overhauser effect (nOe)

The change in intensity of one NMR resonance that occurs when another is saturated is known as the nuclear Overhauser effect (NOE). NOE arises from dipole–dipole cross-relaxation between nuclei, and its usefulness. The strength of a given NOE enhancement is approximately correlated with internuclear separation (actually r⁻⁶ where r is the internuclear distance). However, the NOE also depends on other factors such as molecular motions [148].

In small molecules in solution, the NOE is positive and causes affected resonances to increase in intensity. NOE for small molecules is generally measured using one-dimensional experiments. In small molecules, NOE determins particular stereochemical relationships, such as substitution or ring fusion patterns in largely rigid systems. The NOE is negative for larger molecules and cause affected resonances decrease in intensity. NOE for larger molecules is usually measured using the two-dimensional NOESY experiment or one of its multidimensional variants. Using the NOE to Development of three-dimensional structural information using NOE generally depends on interpretation of an overlapping, redundant network of enhancements, rather than on calibrating precisely the distance dependence of individual enhancements. NOE determine accurate three-dimensional solution structures of biomacromoleculs such as DNA, RNA, or other proteins [149].

A spin-excited nucleus is known to transfer its spin energy to that of an adjacent nucleus resulting in spin relaxation. The efficiency of energy transfer is directly related to the distance between the two nuclei. The nOe grosses advantage of the spin energy transfer [149].

The nOe decreases as the inverse of the sixth power of the distance between the protons. An interesting application of nOe to a structural problem has been described by Hunter et al. [150] When styrene is polymerized in the presence of 4-methoxyphenol, in addition to the polymer, a 1:1 adduct is obtained by the addition of a styrene molecule to 4-methoxyphenol. However, the question of whether the addition occurs at C-2 or C-3 could not be answered from either the ¹H or ¹³C NMR spectrum.

The nOe experiment provided a decision in favour of structure 88. Irradiating the OCH₃ resonance gave an increase in the intensities of the signals of the ring protons H^A and H^B. From this it is obvious that both these protons are ortho to the OCH₃ group. In contrast the signal of the third ring proton H^C showed a negative nOe. This is a case of an indirect nOe in a multi spin system. In further, nOe experiment it was shown that saturating the OH resonance increased the intensity of the H^C signal, providing additional evidence for structure 88.

TWO-DIMENSIONAL NMR SPECTROSCOPY

COSY, a homonuclear 2D NMR correlation spectroscopy, correlates chemical shift of two hydrogen nuclei located on two different carbons that are separated by a single bond via j coupling. Thus it detects the chemical shift for hydrogen’s on both F1 and F2 axis. The most important two-dimensional NMR spectra show either 1h vs 1h or 1h vs 13c chemical shift correlations [126,127]. Here, we attempt to discuss about the some of the important types of 2-D experiments.

Principle

In 2D-NMR, the structural information are obtained from the interactions between two nuclei, either through the bonds which connect them (J-coupling interaction) or directly through space (NOE interaction). These interactions occur at a time by irradiating one resonance in the proton spectrum (either during the relaxation delay or during acquisition) and provide the effect on the intensity or coupling pattern of another resonance. 2D NMR essentially allows us to irradiate all of the chemical shifts in one experiment and gives us a matrix or two-dimensional map of all of the affected nuclei. All possible pairs of nuclei in the sample processed at the same time [128,129].

The basic steps in 2D experiment are as follows.

1. Preparation: Excite nucleus A, creating magnetization in the x-y plane

2. Evolution: Measure the chemical shift of nucleus A.

3. Mixing: Transfer magnetization from nucleus A to nucleus B (via J or NOE).

4. Detection: Measure the chemical shift of nucleus B.

Preparation and Evolution: A 90o pulse excites all of the sample nuclei simultaneously. Detection is simply recording an FID and finding the frequency of nucleus B by Fourier transformation. To get a second dimension, we have to measure the chemical shift of nucleus A before it passes its magnetization to nucleus B. This is accomplished by simply waiting a period of time (called t1, the evolution period) and letting the nucleus A magnetization rotate in the x-y plane. The experiment is repeated many times over (for example, 512 times), recording the FID each time with the delay time t1 incremented by a fixed amount. The time course of the nucleus A magnetization as a function of t1 (determined by its effect on the final FID) is used to define how fast it rotates and thus its chemical shift. Mixing is a combination of RF pulses and/or delay periods which induce the magnetization to jump from A to B as a result of either a J coupling or an NOE interaction (close proximity in space). Different 2D experiments (e.g., NOESY, COSY, HETCOR, etc.) differ primarily in the mixing sequence, since in each one we are trying to define the relationship between A and B within the molecule in a different way [128,129].