Z. Malarski

Isotopic effects in NH…N hydrogen bonds

Abstract The value of the isotopic ratio (ISR) v s (NH)/ v s (ND) has been determined for a number of crystalline compounds containing homoconjugated [NHN] + cations, including various salts of 1,8-bis(N,N-dimethylamino)naphthalene (DMAN). The results obtained, together with the literature data, enabled us to plot a correlation between the ISR and v s (NH) which appears to be similar to that found by Novak for OHO systems. For DMAN · HPF 6 the highest ISR value known so far (2.05) is reported. An empirical analytical form of the correlation is proposed.

Structure and spectroscopic properties of the 1:1 complex of 4-methylpyridine with pentachlorophenol

Abstract The structure of the complex of 4-methylpyridine with pentachlorophenol (MPPCP) has been determined by X-ray diffraction methods. The crystals are triclinic, space group P 1 , with a = 7.408(6), b = 8.934(7), c = 13.653(9) A, α = 100.15(6), β = 118.50(6), γ = 103.67(6)° and Z = 2. The structure solved by the direct methods has been refined to R = 0.026 for 1466 independent reflections. The CO bond length of 1.314(4) a, which is an average literature value for phenol and phenolate bond distances and the O⋯H⋯N hydrogen bond distance of 2.552(4) A, together with IR and UV spectroscopic data, seem to show that the proton in the hydrogen bridge of MPPCP is described by an unsymmetrical double minimum potential energy curve with the vibrational ground level penetrating the top of the barrier.

Structure and IR spectroscopic behaviour of 1,8-bis-(dimethylamino)naphthalene 2,4-dinitroimidazolate

Abstract The crystal structure of 1,8-bis(dimethylamino)naphthalene (DMAN) 2,4-dinitroimidazolate, C 17 H 20 N 6 O 4 , has been determined by X-ray diffraction. The crystals are monoclinic, P2 1 / c , a = 13.426(4), b = 10.465(3), c = 15.915(4) » , β = 126.12(4)°, Z = 4. The structure was solved by direct methods, and refined to an R value of 0.033 for 2291 non-zero independent amplitudes. The [NN⋯N] + bridges of 2.606(3) » with ∠NHN = 160(3)° are characterized by an asymmetric proton density distribution. The IR protonic absorption is located in two regions at about 650 and 1950 cm −1 showing relatively small intensity. The isotopic ratio ν(NHN/ν(NDN) for the low frequency region is almost unity. It seems that hydrogen bonds in protonated DMAN are characterized by a flat asymmetric single minimum potential for the proton motion.

Protonation of 1,8-bis(dimethylamino)naphthalene by various acids in acetonitrile

FTIR studies on a number of systems composed of 1,8-bis(dimethylamino)naphthalene (DMAN) and various organic acids (phenols, benzoic acid derivatives and dicarboxylic acids) were carried out. The degree of protonation was estimated based on the intensity of the Bohlmann bands. In the ionic equilibria the homoconjugated anions play an essential role so that the degree of protonation depends on the excess of acid. The 1:1 complexes of phenols and benzoic acids in acetonitrile form a mixture of protonated DMAN, free DMAN, homoconjugated anions and more complicated species. The broad absorption extending over the whole IR region results from the combination of bands ascribed to ν(NHN+) of protonated DMAN and ν(OHO–) of homoconjugated anions.

Infrared spectra and protonation of 1,8-bis(dimethylamino)naphthalene in acetonitrile

Fourier-transform infrared studies on 1,8-bis(dimethylamino)naphthalene (DMAN) salts in acetonitrile have been performed. In contrast to solid salts very broad ‘continua’ of protonic absorption are observed typical of the usual NHN+ homoconjugated cations formed by the hydrogen bases. The deuterium isotopic ratio ν(NHN)/ν(NDN), estimated from the positions of the band weighted centres, is equal to ca. 1. For some salts a deprotonization of DMANH+ and formation of homoconjugated anions were detected. The tendency to form the homoconjugated anions can be arranged in the sequence C6ClO–5 > NO–3 > NCS– > Cl– > ClO–4. in the case of pentachlorophenolate free DMAN, protonated DMAN and homoconjugated anions are present in the solution in the proportion 1 : 1 : 1.

Ir spectroscopic properties of hydrogen bonding in 1:1 salts of 1,8-bis(N,N-dimethylamino)naphthalene

Abstract Several crystalline salts of 1,8-bis( N,N -dimethylamino)naphthalene (DMAN-“proton sponge”) with acids of varying strength have been isolated. In the crystalline state protonated DMAN is characterized by a band of rich structure at about 500 cm−1, which position depends on the nature of the counter-ion. The isotope ratio for this band is considerably higher than 2 1 2 , which suggests an unusual shape of the potential. The temperature effect is opposite to that expected from the pressure effect, i.e. a temperature decrease shifts the band position towards long wavelengths. In IR spectra of DMAN salts in solution a broad “continous” absorption characteristic for a number of other systems containing the homoconjugated cation [NHN] + appears.

Basicity, IR spectra and protonation of some proton sponges in acetonitrile

Abstract The pK a values in acetonitrile for three proton sponges, 1,8-bis(dimethylamino)naphthalene (DMAN), 1,8-bis(dimethylaminomethyl)naphthalene (DMAMN) and 1,2-bis(dimethylaminomethyl)benzene (DMAMB), were determined. The relations between basicity and structure, and the infrared behaviour of the intramolecular NHN hydrogen bonds formed via protonation by phenols is discussed. It was found that an increase in the basicity of a sponge favours deprotonation of the phenols, while at the same time the strength of the NHN + hydrogen bond diminishes.

New example of a symmetric NHN hydrogen bond in protonated 1,8-bis(dimethylamino)naphthalene (DMAN)

Abstract The crystal structure of the adduct of 1,8-bis(dimethylamino)naphthalene (DMAN) with tetrazole (DMAN-THM), containing crystallographically symmetric NHN + hydrogen bridges, was determined. The crystals of DMAN-THM (monohydrate) are orthorhombic, space group P nma , with Z = 4, a = 9.500(3), b = 10.966(5), c = 15.615(4) A, R (N⋯N) = 2.573(2), r (NH) = 1.312(5) A and ∢ NHN = 157(2)°. The symmetry of the DMAN·H + cations is reflected in an equivalence of the two moieties of the naphthyl ring. In the infrared spectra, the low-frequency ν(NHN) band centred at about 500 cm −1 (half-width of abouth 250 cm −1 ) is observed, typical of very strong NHN hydrogen bonds.

Structure and IR spectroscopic behaviour of NHN hydrogen bonds

Abstract Data related to the geometry of NHN hydrogen bonds are analysed. Particular attention is paid to the homoconjugated NHN + cations which represent potentially symmetric systems. However, only in a few cases was a symmetric proton density distribution found. The shortest NHN bridge reported so far has a length of 2.526(3)A. The analysis of data to date shows that NHN bridges are about 0.10–0.15Alonger than OHO bonds of similar properties in agreement with theoretical predictions. The evolution of IR absorption spectra is to a large extent similar to that of OHO bridges. The structure of broad ν s (NHN) bands can be explained in all detail by taking into account a coupling with δ and γ deformation modes and with their overtones as well as with other internal vibrations which lead to a modulation of the bridge geometry. In the case of protonated “proton sponges” particularly low frequency, relatively narrow ν s (NHN) bands are observed showing anomalous isotopic ratio.

NHN+ hydrogen bonding in salts of 1,8-bis(dimethylaminomethyl)naphthalene (DMAMN)

Abstract Crystals of 1,8-bis(dimethylaminomethyl)naphthalene (DMAMN) pentachlorophenolate were grown from acetonitrile. The space group is C2 c . The unit cell parameters are a = 23.267(5), b = 8.848(2), c = 22.385(5) A , β = 92.73(2)°, Z = 8. The NHN + intramolecular hydrogen bonding distance is 2.609(5) A. The bridge H atom is localized close to the centre of the bridge: r N(1)−H = 1.24(5), r N (2)− H = 1.38(5) A . In the IR absorption spectrum there is a lack of absorption above 1700 cm −1 . Low-frequency broad absorption in the fingerprint region is observed. These results are compared with those reported for other DMAMN salts where hydrogen bonding is markedly longer.