Molecular reorientations of 1-bromo- and 1-iodo-adamantanes1H N.M.R. relaxation study

Unraveling the Complex Solid-State Phase Transition Behavior of 1-Iodoadamantane, a Material for Which Ostensibly Identical Crystals Undergo Different Transformation Pathways

Phase transitions in crystalline molecular solids have important implications in the fundamental understanding of materials properties and in the development of materials applications. Herein, we report the solid-state phase transition behavior of 1-iodoadamantane (1-IA) investigated using a multi-technique strategy [synchrotron powder X-ray diffraction (XRD), single-crystal XRD, solid-state NMR, and differential scanning calorimetry (DSC)], which reveals complex phase transition behavior on cooling from ambient temperature to ca. 123 K and on subsequent heating to the melting temperature (348 K). Starting from the known phase of 1-IA at ambient temperature (phase A), three low-temperature phases are identified (phases B, C, and D); the crystal structures of phases B and C are reported, together with a re-determination of the structure of phase A. Remarkably, single-crystal XRD shows that some individual crystals of phase A transform to phase B, while other crystals of phase A transform instead to phase C. Results (from powder XRD and DSC) on cooling a powder sample of phase A are fully consistent with this behavior while also revealing an additional transformation pathway from phase A to phase D. Thus, on cooling, a powder sample of phase A transforms partially to phase C (at 229 K), partially to phase D (at 226 K) and partially to phase B (at 211 K). During the cooling process, each of the phases B, C, and D is formed directly from phase A, and no transformations are observed between phases B, C, and D. On heating the resulting triphasic powder sample of phases B, C, and D from 123 K, phase B transforms to phase D (at 211 K), followed by the transformation of phase D to phase C (at 255 K), and finally, phase C transforms to phase A (at 284 K). From these observations, it is apparent that different crystals of phase A, which are ostensibly identical at the level of information revealed by XRD, must actually differ in other aspects that significantly influence their low-temperature phase transition pathways. This unusual behavior will stimulate future studies to gain deeper insights into the specific properties that control the phase transition pathways in individual crystals of this material.

Ultrasonic study of 1-X adamantane (X = F, Cl, Br) compounds at high pressure and at order-disorder transitions

We present an ultrasonic study of the elastic properties of 1-X adamantane (X = F, Cl, Br) during order-disorder and order-quasi-order transitions at various temperatures (77-305 K) and high pressures (up to 1 GPa). On the basis of our ultrasonic experiments, we studied for the first time the high-temperature (HT) Fm3m, medium-temperature (MT) P4₂/nmc, and low-temperature (LT) P42₁c phases of 1-fluoroadamantane at high pressures. The elastic properties of these phases at pressures up to 1 GPa at T = 293 and 77 K, as well as at isobaric heating from 77 to 293 K, have been determined. The boundaries of the HT → MT → LT phase transitions have been evaluated, which makes it possible to extend the phase diagram of 1-fluoroadamantane to higher pressures. We have confirmed that the MT → LT transition is a second-order phase transition because it is not accompanied by volume jumps but is manifested in anomalies of the elastic properties and ultrasound transmission both in high-pressure experiments and under isobaric heating. The comparison of the elastic properties of 1-X adamantanes (X = H, F, Cl, Br) has indicated a monotonic dependence at low pressures: the bulk modulus is the highest for adamantane and decreases with an increase of the atomic number of the halogen substitute (from fluorine to bromine).

Phase transitions in 1-bromoadamantane compared to 1-chloroadamantane: similarities and unique features

The elastic properties of 1-chloroadamantane and 1-Bromoadamantane in order-disorder and order-quasi-order phase transitions at temperatures in the range of 77-305 K and high pressures up to 1.1 GPa are studied by the ultrasonic method. The elastic moduli of halogenated adamantanes clearly indicate these transitions, demonstrating high capabilities of the ultrasonic method. Our ultrasonic studies have detected for the first time the λ-anomaly of the elastic properties and, thereby, have confirmed that the phase transition from the orientationally ordered to quasi-ordered phase (M → O) in 1-bromoadamantane is a weak first-order phase transition having some properties of a second-order phase transition. The pressure derivatives of the elastic moduli of 1-chloroadamantane and 1-bromoadamantane in the orientationally ordered phase at 77 K are dB/dP ≈ 8, which allows using the Lennard-Jones potential to theoretically describe this phase.

Nuclear magnetic resonance and dielectric investigations of molecular motions in a glassy crystal: The mixed compound (CN-adm)0.75(Cl-adm)0.25

The dynamic properties of plastic crystalline mixed adamantane's derivatives namely cyanoadamantane (75%) and chloroadamantane (25%) were investigated by dielectric and nuclear magnetic resonance (NMR) spectroscopy, covering a spectral range of 12 decades in the temperature range 110-420 K. Phase transformations were studied and dynamical parameters of the plastic (I), glassy (Ig), and ordered (III) phases were determined and compared with those of pure compounds. The dynamics of the supercooled plastic phase is characterized by an alpha-process exhibiting an Arrhenius behavior which classified the mixed compound as a strong glass former. In the plastic phase, NMR relaxation times were interpreted by using a Frenkel model, which takes into account structural equilibrium positions. This model explains adequately the experimental results by considering two molecular motions. In both the glassy state and plastic phase the motional parameters agree with those of 1-cyanoadamantane. On the contrary, in the ordered phase, the motional parameters related to the uniaxial rotation of chloroadamantane molecules indicate an accelerated motion.

A high pressure FT-IR spectroscopic study of phase transitions in 1-chloro- and 1-bromoadamantane

The infrared spectra of two orientationally disordered adamantane derivatives, 1-chloro- and 1-bromoadamantane, have been obtained as a function of pressure by use of a diamond anvil cell. Phase transitions were detected from the splittings of vibrational bands and from changes in the slopes on plots of frequency versus pressure. In 1-chloroadamantane, a disorder-order phase transition was detected at approximately 5 kbar. This high-pressure phase is identical to the ordered phase obtained at low temperature. A second phase transition at high pressure was identified, for the first time, at approximately 17 kbar. The new high-pressure phase is an ordered phase. For 1-bromoadamantane, a semi-ordered to ordered phase transition was detected at about 5 kbar, and no other transitions were observed up to a pressure of 35 kbar. For both derivatives, the phase changes were reversible.