3 research outputs found
One Hydrogen BondTwo Ways To Build a Structure. The Role of N–H···O Hydrogen Bonds in Crystal Structures of <i>N</i>,<i>N</i>‑Dimethylglycine
Crystal
structures of amino acids are considered to mimic important
interactions in peptides; therefore the studies of the structure-forming
factors in these systems attract much attention. <i>N</i>,<i>N</i>-Dimethylglycine is an interesting model compound
that was used to test the role of the N–H···O
hydrogen bonds in forming the head-to-tail chains, the main structural
unit in the crystals of amino acids. It was hypothesized previously
[Kolesov, B. A.; Boldyreva, E. V. J. Raman Spectrosc. 2010, 41, 670−677] that additional side N–H···O
hydrogen bonds play an important role in forming the head-to-tail
chains of amino acid zwitterions linked via N–H···O
hydrogen bonds between the charged −NH<sub>3</sub><sup>+</sup> and −COO<sup>–</sup> terminal groups. The twice methylated
amino group of <i>N</i>,<i>N</i>-dimethylglycine
is able to form only one N–H···O hydrogen bond
in the crystal structure, so this hypothesis could be tested. In the
present article, we describe the crystal structures of two polymorphs
of <i>N</i>,<i>N</i>-dimethylglycine, in which
the zwitterions are packed in two different ways. In one polymorph
(orthorhombic, <i>Pbca</i>), they form finite four-membered
ring motifs not linked to each other via any hydrogen bonds but only
by weak van der Waals interactions. However, in the second polymorph
(monoclinic, <i>P</i>2<sub>1</sub>/<i>n</i>, which
was never described before), the zwitterions do form infinite head-to-tail
chains though the N–H···O bond is the only interaction
and is not assisted via any additional hydrogen bonds. The effect
of cooling on the two crystal structures was followed by single-crystal
X-ray diffraction combined with polarized Raman spectroscopy of oriented
single crystals, in order to compare the response of the N–H···O
bonds to temperature variations. The crystal structure of the monoclinic
polymorph with infinite chain motifs compresses anisotropically on
cooling, whereas that of the orthorhombic polymorph with finite ring
motifs undergoes a reversible single-crystal to single-crystal phase
transition at ∼200 K accompanied by nonmerohedral twinning,
reducing the space symmetry to monoclinic (<i>P</i>2<sub>1</sub>/<i>b</i>) and doubling the asymmetric unit from
two to four molecules. This phase transition could not be detected
by Raman spectroscopy and DSC. The temperature dependent structure
and relative stability of both polymorphs were studied by periodic
DFT calculations. The monoclinic polymorph appears to be more stable
(by 0.8–1.2 kcal/mol, depending on the density), but with the
increasing density and decreasing temperature, the difference decreases.
The phase transition of the orthorhombic polymorph has no detectable
impact on its relative stability
One Hydrogen BondTwo Ways To Build a Structure. The Role of N–H···O Hydrogen Bonds in Crystal Structures of <i>N</i>,<i>N</i>‑Dimethylglycine
Crystal
structures of amino acids are considered to mimic important
interactions in peptides; therefore the studies of the structure-forming
factors in these systems attract much attention. <i>N</i>,<i>N</i>-Dimethylglycine is an interesting model compound
that was used to test the role of the N–H···O
hydrogen bonds in forming the head-to-tail chains, the main structural
unit in the crystals of amino acids. It was hypothesized previously
[Kolesov, B. A.; Boldyreva, E. V. J. Raman Spectrosc. 2010, 41, 670−677] that additional side N–H···O
hydrogen bonds play an important role in forming the head-to-tail
chains of amino acid zwitterions linked via N–H···O
hydrogen bonds between the charged −NH<sub>3</sub><sup>+</sup> and −COO<sup>–</sup> terminal groups. The twice methylated
amino group of <i>N</i>,<i>N</i>-dimethylglycine
is able to form only one N–H···O hydrogen bond
in the crystal structure, so this hypothesis could be tested. In the
present article, we describe the crystal structures of two polymorphs
of <i>N</i>,<i>N</i>-dimethylglycine, in which
the zwitterions are packed in two different ways. In one polymorph
(orthorhombic, <i>Pbca</i>), they form finite four-membered
ring motifs not linked to each other via any hydrogen bonds but only
by weak van der Waals interactions. However, in the second polymorph
(monoclinic, <i>P</i>2<sub>1</sub>/<i>n</i>, which
was never described before), the zwitterions do form infinite head-to-tail
chains though the N–H···O bond is the only interaction
and is not assisted via any additional hydrogen bonds. The effect
of cooling on the two crystal structures was followed by single-crystal
X-ray diffraction combined with polarized Raman spectroscopy of oriented
single crystals, in order to compare the response of the N–H···O
bonds to temperature variations. The crystal structure of the monoclinic
polymorph with infinite chain motifs compresses anisotropically on
cooling, whereas that of the orthorhombic polymorph with finite ring
motifs undergoes a reversible single-crystal to single-crystal phase
transition at ∼200 K accompanied by nonmerohedral twinning,
reducing the space symmetry to monoclinic (<i>P</i>2<sub>1</sub>/<i>b</i>) and doubling the asymmetric unit from
two to four molecules. This phase transition could not be detected
by Raman spectroscopy and DSC. The temperature dependent structure
and relative stability of both polymorphs were studied by periodic
DFT calculations. The monoclinic polymorph appears to be more stable
(by 0.8–1.2 kcal/mol, depending on the density), but with the
increasing density and decreasing temperature, the difference decreases.
The phase transition of the orthorhombic polymorph has no detectable
impact on its relative stability
Structural Properties of Nickel Dimethylglyoxime at High Pressure: Single-Crystal X‑ray Diffraction and DFT Studies
Structural
changes in nickel dimethylglyoxime (Ni(dmg)<sub>2</sub>) were followed
by single-crystal X-ray diffraction in a diamond-anvil
cell (DAC) at pressures up to 5.1 GPa, that is, in the pressure range
through the major color change point (2 GPa), but before the phase
transition at 7.4 GPa. Significant average compression (∼4%/GPa)
was observed, with anisotropic, but continuous and monotonic lattice
strain. The maximum compression was observed for the direction perpendicular
to planar layers of Ni(dmg)<sub>2</sub> and thus corresponds to decreasing
the shortest contacts between nickel cations. Compression within the
layers was not so pronounced as the compression between the layers.
The structure and dynamics of the short O–H···O
hydrogen bond connecting the adjacent dimethylglyoxime ligands were
investigated by periodic DFT calculations and showed evidence of a
flat, asymmetric single-well proton potential facilitating large-amplitude
proton oscillations. The proton motion appears to be coupled to the
dynamics of the adjacent methyl groups, resulting in the increased
asymmetry of the hydrogen bond at higher pressures