The Bond‐Rotational Mobility of Guanidinium Ion

Bally, Thomas; Diehl, P.; Haselbach, Edwin; Tracey, Alan S.

doi:10.1002/hlca.19750580819

Cited by 15 publications

(8 citation statements)

References 12 publications

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“…The equivalence of the N1 and N3 chemical shifts indicates that rotation about the C-N2 bond in these compounds is fast on the NMR time scale. This observation is in accord with studies by Bally and coworkers (9) and also by Kessler and Leibfritz (10), who have shown that the activation energy for rotation about the C-N2 bond is 10-13 kcal/mol (42-54 kJ/mol) for closely related guanidinium analogs. When the shifts in Table 2 are compared with the 15N shifts of N-arylguanidines themselves (unpublished results), it is evident that protonation produces a large diamagnetic shift of -50 ppm in the N2 resonances and a somewhat smaller paramagnetic shift of -8 ppm in the N1 and N3 resonances.…”

Section: Nh2supporting

confidence: 81%

Substituent effects on the nitrogen-15 and carbon-13 shieldings of some N -arylguanidinium chlorides

Botto

Schwartz²,

Roberts³

1980

Proc. Natl. Acad. Sci. U.S.A.

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The 13C and 15N chemical shifts of five Narylguanidinium chlorides carrying polar substituents, ranging in character from 4-methoxy to 4-nitro groups, have been determined by NMR spectroscopy at the natural-abundance level of 13C and 15N in dimethyl sulfoxide solution. Comparison of the 13C shifts of these salts with those of monosubstituted benzenes shows that the guanidinium group induces an average downfield shift of -5.8 ppm of the resonance of the aryl carbon to which it is attached (Cl), an average upfield shift of +4.2 ppm for C2 and C6, and a small unfield shift of +1.9 ppm for C4. The shifts of C3 and C5 are smal and erratic relative to the corresponding carbons in monosubstituted benzenes. The 15N resonances of the guanidinium nitrogens are quite sensitive to electric effects resulting from substitution of polar groups at C4.The 15N shift of the =NAr nitrogen relative to that of the salts suggests that the predominant tautomer for N-arylguanidines is (H2N)2C=NAr. The 15N shifts of the (NH2)2 nitrogens correlate rather well with ap -parameters, whereas the shifts of the -NHAr nitrogens seem to correlate only with R values derived from the ap -substituent constants.Guanidine is the strongest known organic base, its remarkable basicity being ascribed to substantially increased electron delocalization made possible by addition of a proton (for review, see ref. 1). To assess the influence of polar substituents on the "3C and "5N NMR shifts of guanidinium ions, we have investigated the effects produced by a substituent, X, for several N-arylguanidinium chlorides for which the resonance structures 1-3 are usually written to account for their reluctance to lose a proton relative to ammonium salts. Table 1. The resonances of the ring carbons were assigned on the basis of shift data for the corresponding substituted benzenes (2, 3) and N-(arylmethylidene)amines (4) as well as spin-spin splittings in the coupled spectra. There is a reasonable parallelism between the shifts of most of the ring carbons and those of the corresponding carbons in monosubstituted benzenes (2, 3). For CI, there is an average deshielding of -5.8 I 0.6 ppm; for C2 and C6 the average shielding is 4.2 + 0.4 ppm, and for C4 the average shielding is 1.9 ± 1.1 ppm resulting from substitution of a guanidinium group. The effects at C3 and C5 are relatively smaller and erratic.The C7 chemical shifts are virtually insensitive toward substitution of polar groups at C4. This is surprising, considering the sensitivity of 3-carbon shieldings in 4-substituted cyclopropylbenzenes (5), 4-substituted ethenylbenzenes (6), and N-(benzylidene)arenamines (7) and the /-nitrogen shieldings of N-(arylmethylidene)amine hydrochlorides (8). Apparently, the electrical effects of the polar groups, X, are transmitted primarily to the nitrogens of the guanidinium groups because, as we will show, the effects are much larger on the 5N chemical shifts than on the "3C shifts of the C7 carbons.The 'IN chemical shifts of the N-arylguanidinium chlorides are given in...

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Section: Nh2supporting

confidence: 81%

Substituent effects on the nitrogen-15 and carbon-13 shieldings of some N -arylguanidinium chlorides

Botto

Schwartz²,

Roberts³

1980

Proc. Natl. Acad. Sci. U.S.A.

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“…The amino rotational barriers for the global minima (complexes 1A and 2B ) are summarized in Table . It can be observed that these rotational barriers are significantly higher than those previously reported for the corresponding neutrals. At similar levels of accuracy the amino rotational barrier of acetamidine is 9.5 kcal/mol, while that of guanidine is only 5.2 kcal/mol.…”

Section: Resultscontrasting

confidence: 62%

“…An increase in the rotational barriers for both compounds is also observed upon protonation. Protonated acetamidine was predicted to have a rotational barrier of about 21 kcal/mol, while that of guanidinium ion is about 13 kcal/mol . However, the origin of this increase is not exactly the same.…”

Section: Resultsmentioning

confidence: 99%

Acetamidine−X⁺and Guanidine−X⁺(X = Li, Na, Mg, Al) Complexes in the Gas-Phase. A Theoretical Study

et al. 1997

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The structures, harmonic vibrational frequencies, amino rotational barriers and binding energies of complexes formed by Li+, Na+, Mg+, and Al+ association to guanidine and acetamidine have been investigated by means of the B3LYP density functional approach. Both neutrals are predicted to be stronger bases than ammonia when the reference acids are the aforementioned metal cations. The basicity enhancement with respect to Mg+ and Al+ is only slightly smaller than that reported before when the reference acid is H+. Metal cation association leads to a significant increase in the corresponding amino rotational barriers and to sizeable shiftings of different vibrational modes, in particular those associated with the amino groups. For Li+ and Na+ complexes, the ion−neutral interactions are essentially electrostatic, while the bonds involving Al+ have a significant covalent character. For this reason the Al+ complexes closely resemble the corresponding protonated species. Mg+ represents an intermediate situation between alkali cations and Al+. The empty 3p orbitals of the metal cation play an important role in this respect.

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“…Barriers to rotation about the C-N bond are expected to be much higher than in materials such as amines where the n-bonding is not present. The barriers obtained in the ab initio calculations (1 -4) differ among themselves, but from one nmr study of the guanidinium ion in liquid crystal solution a barrier of 13 kcal/mol was reported (1). This value seems rather low, however, when compared to the barriers reported from nmr studies of formamide and acetamide in solution, which are upwards of 16 kcal/mol (e.g.…”

Section: Introductionmentioning

confidence: 84%

Nuclear magnetic resonance studies of molecular motion in guanidinium chloride, bromide, and iodide

Ratcliffe¹

1985

Can. J. Chem.

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Guanidinium [Formula: see text] chloride, bromide, and iodide salts have been studied by 1H and 2H nmr as a function of temperature. The 2H powder lineshapes show conclusively that reorientation of the guanidinium ion occurs about the principal three-fold axis in the chloride, bromide, and high temperature phase of the iodide. Activation energies for this process have been obtained from 1H spin-lattice relaxation results. The question of whether or not there are concurrent two-fold flips of the —NH2 units is discussed, but must presently remain unresolved. It was found that the high temperature phase of the iodide can be supercooled.

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The Bond‐Rotational Mobility of Guanidinium Ion

Cited by 15 publications

References 12 publications

Substituent effects on the nitrogen-15 and carbon-13 shieldings of some N -arylguanidinium chlorides

Substituent effects on the nitrogen-15 and carbon-13 shieldings of some N -arylguanidinium chlorides

Acetamidine−X⁺and Guanidine−X⁺(X = Li, Na, Mg, Al) Complexes in the Gas-Phase. A Theoretical Study

Nuclear magnetic resonance studies of molecular motion in guanidinium chloride, bromide, and iodide

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The Bond‐Rotational Mobility of Guanidinium Ion

Cited by 15 publications

References 12 publications

Substituent effects on the nitrogen-15 and carbon-13 shieldings of some N -arylguanidinium chlorides

Substituent effects on the nitrogen-15 and carbon-13 shieldings of some N -arylguanidinium chlorides

Acetamidine−X+and Guanidine−X+(X = Li, Na, Mg, Al) Complexes in the Gas-Phase. A Theoretical Study

Nuclear magnetic resonance studies of molecular motion in guanidinium chloride, bromide, and iodide

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Product

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Acetamidine−X⁺and Guanidine−X⁺(X = Li, Na, Mg, Al) Complexes in the Gas-Phase. A Theoretical Study