T1 Minimum Research Articles

Comprehensive NMR Study of Magnesium Borohydride

1H and 11B NMR measurements were performed to study BH4 reorientations and diffusion in four samples of Mg(BH4)2. These include the low (α) and high (β) temperature bulk solid phases, α-phase material with TiF3 and ScCl3 additives, and α-phase material incorporated into a carbon aerogel. All four samples show 1H T1 minima within our temperature range from reorientational motion of the BH4 tetrahedra. The β-phase 1H T1 minimum occurs at a lower temperature than that for the α-phase, indicating a lower activation energy for reorientations. The α-phase 1H spectra display the onset of broadening at low temperatures because of the slowing of reorientations but show no indication of line narrowing at high temperatures from translational diffusion. The β-phase 1H spectra are similar, but here T1D measurements were performed to determine the BH4 hopping rate. Above 150 °C, a significant narrow component does appear in the 1H spectra of the aerogel sample, indicating substantially increased diffusive motions in th...

1H NMR Studies on the Cation Motions in Crystalline Cadmium Bromide Complexes [C(NH2)3]Cd2Br5, [(CH3)3NH]3Cd2Br7, and [i-C3H7NH3]CdBr3

1H NMR T1 measurements of crystalline [C(NH2)3]Cd2Br5 showed a single minimum due to the C3 reorientation of the planar [C(NH2)3]+ ion with an activation energy (Ea) of 35.8 kJ mol−1. In [(CH3)3NH]3Cd2Br7 crystals, two T1 minima appeared which are assigned to the C3 reorientation of methyl groups in the [(CH3)3NH]+ cation with Ea = 13.0 kJ mol−1 and to the C3 reorientation of a whole cation around the molecular C3 axis with Ea = 28.9 kJ mol−1. In [i-C3H7NH3]CdBr3 crystals, a very broad T1 minimum appeared near 160 K which is assigned to the C3 reorientations of two methyl groups with Ea = 11.3 kJ mol−1 and of an NH3 group with Ea = 13.3 kJ mol−1 in the [i-C3H7NH3]+ ion, and another minimum with Ea = 23.5 kJ mol−1 near 300 K assigned to a higher order molecular motion of the cation.

Time course of cardiovascular responses induced by mental and orthostatic challenges

Cardiovascular responses to single stressors diminish over time. Interaction of different stressors influencing hemodynamic variables, indicative of stress-induced reactivity and physiological responses are, however, poorly understood. We investigated time course of mental (using mental arithmetic, MA) and orthostatic (using head up tilt, HUT) challenges induced responses in 16 males. Three protocols were used: HUT, MA and MA + HUT, with sessions randomized and two weeks apart. Hemodynamic responses were compared for 30 s epochs of stress application (stress T1, stress T2…). Compared to baseline, HUT, HUT + MA and MA applications affected heart rate (HR) (+ 15.1 ± 8.0 bpm, + 20.0 ± 9.2 bpm, + 11.9 ± 7.2 bpm, all p's < .001, respectively) and stroke volume (SV) (− 22.3 ± 8.1 ml, − 22.0 ± 10.4 ml, − 7.6 ± 8.7 ml, all p's < .001, respectively). HUT and MA + HUT induced HR increases were higher in stress T2 compared to stress T1 ( p < .05) and reached maximum at stress T2. HUT and MA + HUT further reduced SV in stress T2 as compared to stress T1 ( p < .001); lowest SV was in stress T2. Mean arterial pressure reached its minimum in stress T1 during HUT and MA + HUT (− 6.0 ± 8.5 mm Hg, p < .001, − 4.4 ± 9.7 mm Hg, p < .01, respectively) but increased in MA (+ 4.3 ± 3.7 mm Hg, p < .01). Combination of MA + HUT resulted in different time courses of blood pressure responses as compared to HUT alone. We conclude that application of single or combined stress challenges lead to stressor- and time dependent-initial changes in cardiovascular responses. Our findings provide novel insights regarding the duration a stressor must be applied to elicit maximal cardiovascular responses.

Feasibility of 19F-NMR for Assessing the Molecular Mobility of Flufenamic Acid in Solid Dispersions

The purpose of the present study was to clarify the feasibility of 19F-NMR for assessing the molecular mobility of flufenamic acid (FLF) in solid dispersions. Amorphous solid dispersions of FLF containing poly(vinylpyrrolidone) (PVP) or hydroxypropylmethylcellulose (HPMC) were prepared by melting and rapid cooling. Spin-lattice relaxation times (T1 and T(1rho)) of FLF fluorine atoms in the solid dispersions were determined at various temperatures (-20 to 150 degrees C). Correlation time (tauc), which is a measure of rotational molecular mobility, was calculated from the observed T1 or T1rho value and that of the T1 or T1rho minimum, assuming that the relaxation mechanism of spin-lattice relaxation of FLF fluorine atoms does not change with temperature. The tauc value for solid dispersions containing 20% PVP was 2-3 times longer than that for solid dispersions containing 20% HPMC at 50 degrees C, indicating that the molecular mobility of FLF in solid dispersions containing 20% PVP was lower than that in solid dispersions containing 20% HPMC. The amount of amorphous FLF remaining in the solid dispersions stored at 60 degrees C was successfully estimated by analyzing the solid echo signals of FLF fluorine atoms, and it was possible to follow the overall crystallization of amorphous FLF in the solid dispersions. The solid dispersion containing 20% PVP was more stable than that containing 20% HPMC. The difference in stability between solid dispersions containing PVP and HPMC is considered due to the difference in molecular mobility as determined by tauc. The molecular mobility determined by 19F-NMR seems to be a useful measure for assessing the stability of drugs containing fluorine atoms in amorphous solid dispersions.

NMR Study of the Molecular Dynamics of D-Amphetamine Sulfate Salt Powder

The temperature dependence of the proton spin–lattice relaxation times was measured for the D-amphetamine sulfate salt in the temperature range from 80 to 466 K. Two minima of T 1 were observed and explained in terms of reorientation of the CH3 and NH3 groups. The T 1 minimum attributed to the motion of methyl groups was analyzed assuming two dynamically inequivalent types of these groups in the unit cell of the studied salt. The activation parameters for the C 3 reorientation of the methyl and ammonium groups were determined. A structural phase transition was evidenced at about 335 K in the polycrystalline sample of the D-amphetamine sulfate salt.

Study of molecular dynamics and phase transitions in trimethylammonium trichlorogermanate using 1H NMR and DSC measurements

Abstract(CH3)3NHGeCl3 is prepared, characterized and studied by 1H NMR and differential scanning calorimetry to understand the molecular dynamics and phase transitions. The proton spin lattice relaxation time (T1) is measured at 20.53 MHz in the temperature range 391–5.2 K employing a home‐made pulsed NMR spectrometer. The T1 results show that (CH3)3NHGeCl3 undergoes several phase transitions in the temperature range 391–50 K. Both T1 data and DSC thermograms support the occurrence of phase transitions at 272 K and around 323 K. T1 data are analyzed in three temperature regions using standard models. The T1 variations in the intermediate temperature region 272–100 K are attributed to the hindered reorientations of symmetric groups (CH3 and (CH3)3NH). A broad asymmetric T1 minimum in the temperature region 45–20 K is attributed to quantum rotational tunneling of the methyl groups. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Reorientational Dynamics and Solid-Phase Transformation of Ammonium Dicyanamide into Dicyandiamide: A 2H Solid-State NMR Study

The reorientational dynamics of ammonium dicyanamide ND4[N(C[triple bond]N)2] and the kinetics as well as the mechanism of the solid-state isomerization reaction from ammonium dicyanamide into dicyandiamide (N[triple bond]C-N==C(NH2)2) was studied by means of 2H and 14N solid-state NMR spectroscopy in a temperature range between 38 and 390 K. Whereas in previous investigations the mechanism of the solid-state transformation was investigated by means of vibrational and magic angle spinning solid-state NMR spectroscopy as well as neutron diffraction, we here present a comprehensive 2H study of the ammonium ion dynamics prior to and during the course of the reaction, thereby highlighting possible cross correlations between dynamics and reactivity involving the ammonium ion. The ND4+ group was found to undergo thermally activated random jumps in a tetrahedral potential, which is increasingly distorted with increasing temperature, giving rise to an asymmetrically compressed or elongated tetrahedron with deviations from the tetrahedral angle of up to 6 degrees . The correlation time follows an Arrhenius law with an activation energy of Ea = 25.8(2) kJ mol(-1) and an attempt frequency of tau0(-1) = 440(80) THz. The spin-lattice relaxation times were fitted according to a simple Bloembergen-Purcell-Pound type model with a T1 minimum of 4 ms at 230 K. Temperature-dependent librational amplitudes were extracted by line-shape simulations between 38 and 390 K and contrasted with those obtained by neutron diffraction, their values ranging between 5 and 28 degrees . The onset and progress of the solid-phase transformation were followed in situ at temperatures above 372 K and could be classified as a strongly temperature-dependent, heterogeneous two-step reaction proceeding with rapid evolution of ammonia and comparatively slow subsequent reintegration into the solid. On the microscopic level, this correlates with a rapid proton transfer -- possibly triggered by a coupling between the ammonium ion dynamics and phonon modes on the terahertz time scale -- and an essentially decoupled nucleophilic attack of ammonia at the nitrile carbon, giving rise to significantly differing time constants for the two processes.

Nuclear Magnetic Resonance Study of the Dynamics of Imidazolium Ionic Liquids with −CH2Si(CH3)3 vs −CH2C(CH3)3 Substituents

Trimethylsilylmethyl (TMSiM)-substituted imidazolium bis(trifluoromethylsulfonyl)imide (NTf(2)-), and tetrafluoroborate (BF(4)-) ionic liquids (ILs) have lower room-temperature viscosities by factors of 1.6 and 7.4, respectively, than isostructural neopentylimidazolium ILs. In an attempt to account for the effects of silicon substitution in imidazolium RTILs and to investigate the ion dynamics, we report nuclear magnetic resonance (NMR) measurements of 1H (I = 1/2) and 19F (I = 1/2) spin-lattice relaxation times (T1) and self-diffusion coefficients (D) as a function of temperature for ILs containing the TMSiM group and, for comparison, the analogous neopentyl group. The 1H and 19F nuclei probe the dynamics of the cations and anions, respectively. The low-temperature line shapes were determined to be Gaussian, and the onset of the rigid lattice line width is correlated with the measured glass transition temperature. The spin-lattice relaxation data feature a broad T1 minimum as a function of inverse temperature for both nuclear species. The Arrhenius plots of the diffusion data for both nuclear species are found to exhibit Vogel-Tammann-Fulcher curvature. Analysis of the eta and D data generally show fractional Stokes-Einstein behavior D proportional to (T/eta)m. This is most prominent in the neopentylimidazolium BF(4)- IL with m approximately 0.66.

Application of Schrödinger Equation to Study the Tunnelling Dynamics of Proton Transfer in the Hydrogen Bond of 2,5-Dinitrobenzoic Acid: Proton T1, T1ρ, and Deuteron T1 Relaxation Methods

Temperature measurements of proton T1 (24.7 MHz), deuteron (deuterated hydroxyl group) T1 (55.2 MHz), and proton T1(rho) (B1 = 9 G) spin-lattice relaxation times of 2,5-dinitrobenzoic acid have been performed. An analysis of present experimental data together with previously published proton T1 (55.2 MHz) data has revealed the following molecular motions: proton/deuteron transfer in the hydrogen bond and two-site hopping of the whole dimer. It is shown that the proton-transfer dynamics are characterized by two correlation times tau(ov) and tau(tu), describing two fundamentally different motional processes, namely, thermally activated jumps over the barrier and tunneling through the barrier. The temperature dependence of 1/tau(tu) is the solution of Schrödinger's equation, which also yields the temperature T(tun), where begins the tunnel pathway for proton transfer. A new equation for the spectral density function of complex motion consisting of the three motions is derived. The third motion (two-site hopping of the whole dimer characterized by tau(lib) correlation time) is responsible for a proton T1(rho) minimum in high temperatures, just below the melting point. Such a minimum is not reached by T1 temperature dependencies. The minimum of T1(rho) assigned to the classical hopping of a hydrogen-bonded proton occurs in the same low-temperature regime in which the flattening of the temperature dependencies of T1 points to the dominance of incoherent tunneling. This experimental fact denies the known theories predicting the intermediate temperature regime where a smooth transition between classical and quantum tunneling dynamics is expected. The fit of the derived theoretical equations to the experimental data T1(rho) and T1 is satisfactory. The correlation times obtained for deuterons indicate deuteron-transfer dynamics much slower than proton-transfer dynamics. It is concluded that the classical proton transfer takes place over the whole temperature regime, while the incoherent tunneling occurs below 46.5 (hydrogen) or 87.2 K (deuterium) only.

Reorientational Dynamics and Solid-Phase Transformation of Ammonium Dicyanamide into Dicyandiamide

The reorientational dynamics of ammonium dicyanamide ND4[N(C≡N)2] and the kinetics as well as the mechanism of the solid-state isomerization reaction from ammonium dicyanamide into dicyandiamide (N≡C-N=C(NH2)2) was studied by means of 2H and 14N solid-state NMR spectroscopy in a temperature range between 38 and 390 K. Whereas in previous investigations the mechanism of the solid-state transformation was investigated by means of vibrational and magic angle spinning solid-state NMR spectroscopy as well as neutron diffraction, we here present a comprehensive 2H study of the ammonium ion dynamics prior to and during the course of the reaction, thereby highlighting possible cross correlations between dynamics and reactivity involving the ammonium ion. The ND4+ group was found to undergo thermally activated random jumps in a tetrahedral potential, which is increasingly distorted with increasing temperature, giving rise to an asymmetrically compressed or elongated tetrahedron with deviations from the tetrahedral angle of up to 6°. The correlation time follows an Arrhenius law with an activation energy of Ea = 25.8(2) kJ mol-1 and an attempt frequency of τ0-1 = 440(80) THz. The spin−lattice relaxation times were fitted according to a simple Bloembergen−Purcell−Pound type model with a T1 minimum of 4 ms at 230 K. Temperature-dependent librational amplitudes were extracted by line-shape simulations between 38 and 390 K and contrasted with those obtained by neutron diffraction, their values ranging between 5 and 28°. The onset and progress of the solid-phase transformation were followed in situ at temperatures above 372 K and could be classified as a strongly temperature-dependent, heterogeneous two-step reaction proceeding with rapid evolution of ammonia and comparatively slow subsequent reintegration into the solid. On the microscopic level, this correlates with a rapid proton transferpossibly triggered by a coupling between the ammonium ion dynamics and phonon modes on the terahertz time scaleand an essentially decoupled nucleophilic attack of ammonia at the nitrile carbon, giving rise to significantly differing time constants for the two processes.

Arthur A. Demarest, The Petexbatun Regional Archaeological Project. A Multidisciplinary Study of the Maya Collapse. (Nashville: Vanderbilt University Press (Vanderbilt Institute of Mesoamerican Archaeology Series, Volume 1), 2006, 211 pp., 56 figs, 3 colour plates, hbk, ISBN 0 8265 1443 X)

Seventy-five ASA Grades I-III patients (18-85 years, 45-90 kg) were randomized into five groups. All patients received N2O/O2 (2/1) and alfentanil: loading dose (LD) 0.015 mg kg-1 and maintenance dose (MD) 0.045 mg kg-1 h-1 (groups 1-4). Group 1 received propofol (LD 2 mg kg-1 and MD 6 mg kg-1 h-1); Group 2 etomidate (LD 0.3 mg kg-1 and MD 0.6 mg kg-1 h-1); Group 3 midazolam (LD 0.2 mg kg-1 and MD 0.120 mg kg-1 h-1); Group 4 methohexitone (LD 1.5 mg kg-1 and MD 4 mg kg-1 h-1); Group 5 dehydrobenzperidol 0.05-0.23 mg kg-1 and alfentanil (LD 0.100 mg kg-1 and MD 0.060 mg kg-1 h-1). The neuromuscular block induced by pipecuronium (50 micrograms kg-1) was evaluated. No statistically significant differences were found between the five groups as concerned degree of block (expressed as % twitch amplitude in response to the first of the TOF stimuli (Ta1) at intubation, T1 minimum and recovery to Ta1 = 20%, 25% and 75%. Slightly faster intubation was possible when midazolam was used in comparison with propofol, methohexitone or NLA and when etomidate was used in comparison with propofol. A wide range of individual values of maximal neuromuscular blocking activity was found.

Late magnetic resonance imaging features of leukoencephalopathy in children with central nervous system tumours following high-dose methotrexate and neuraxis radiation therapy

High-dose methotrexate (HDMTX) is used increasingly to treat children with central nervous system (CNS) tumours. Although the neuro-imaging features of leukoencephalopathy associated with systemic or intrathecal methotrexate administered after cranial radiation have been well described, the extent to which the sequencing of HDMTX prior to cranial radiation in infants and children predisposes to late neuroradiological features of leukoencephalopathy is unknown. This report describes the National Cancer Institute (NCI) toxicity grade of leukoencephalopathy based on magnetic resonance imaging (MRI) findings in all patients who survived 4 or more years after treatment on an earlier phase II study. These patients, with newly diagnosed CNS embryonal tumours, were in the age range 3.5–14.2 years (median 6.9 years) at diagnosis, and received four courses of pre-irradiation combination chemotherapy, including HDMTX 8 g/m 2. Following completion of the ‘up-front’ phase II study, all patients received conventionally fractionated whole brain doses of 36–50.4 Gy. The radiation dose and treatment volumes were determined individually according to the primary tumour location and results of extent of disease evaluations. The most recent MRI brain scans, obtained 4.0–10.5 years (median 6.5 years) after radiation therapy and comprising a minimum of T1, T1 following gadolinium and T2 sequences, were reviewed centrally to assess the neuroradiological grade of leukoencephalopathy, based on the NCI Common Terminology Criteria for Adverse Events, v3.0. Grade I changes (mild increase in subarachnoid space, and/or mild ventriculomegaly, and/or small/focal T2 hyperintensities) were evident in 8 of the 12 patients and grade II changes (moderate increase in subarachnoid space and/or moderate ventriculomegaly, and/or focal T2 hyperintensities extending to the centrum ovale) were found in the remaining 4. In conclusion, treatment with multiple courses of HDMTX prior to 36–50.4 Gy cranial radiation did not result in moderate to severe MRI features of leukoencephalopathy. Future studies in paediatric neuro-oncology patients, involving HDMTX combined with prospective neuropsychological evaluations appear justified.

Thermal conductivity and 89Y NMR of Ca2+xY2$minus;xCu5O10

A two-peak feature in the thermal conductivity, κ(T), and 89Y NMR results, including a shallow minimum of T1, indicate strong copper spin correlations in the spin-chain compound Ca2+xY2−xCu5O10 at temperatures well above the previously assumed exchange coupling strength, |J/kB|=25–80K.

PROTON CORRELATION TIMES AND SEGMENTAL MOBILITY OF HUMIC ACIDS IN TWO SOLVENTS

Molecular dynamics of humic acid in solution are rarely investigated. Thus, the objective of this study is to determine proton correlation time and segmental mobility of humic acids in solution. Proton spin-lattice relaxation time (T1) was determined for humic acids in DMSO-d6 and 0.5 M NaOD at NMR frequencies of 300 MHz and 600 MHz; the correlation times were then estimated for aliphatic protons, protons associated with oxygen-containing functionalities, and aromatic protons. Results suggest that proton correlation times of humic acids in solvents were in the order of 10−10 seconds, which were near the T1 minimum at 600 MHz. Carbohydrate was identified as the largest and the most immobile component in humic acids, whereras aliphatic and aromatic fragments were relatively smaller and more mobile. The relaxation measurement is a useful approach for investigating the segmental mobility of humic acids in aqueous solution.

Thermal conductivity and NMR of Ca2+Y2−Cu5O10

Abstract A two-peak feature in the thermal conductivity, κ(T), and 89 Y NMR results, including a shallow minimum of T1, indicate strong copper spin correlations in the spin-chain compound Ca2+xY2−xCu5O10 at temperatures well above the previously assumed exchange coupling strength, |J/kB|=25– 80 K .

Molecular Dynamics in a Chiral Smectic Liquid Crystal Studied by 13C-Nuclear Magnetic Resonance Spin-Lattice Relaxation Time Observation

In order to study molecular rotation around its long axis in chiral smectic phases, the temperature dependence of the spin-lattice relaxation time T1 is investigated in an antiferroelectric liquid crystal, (S)-4-(1-methylheptyloxy-carbonyl)phenyl 4'-octyloxybiphenyl-4-carboxylate (S-MHPOBC) by means of solid-state 13C-NMR spectroscopy. The spin-lattice relaxation time T1 of the aromatic core part has a minimum at about 95°C in its temperature dependent curve. Since T1 minimum occurs under a condition of ω0τc∼1, the correlation time τc of molecular motion should be of the order of 2π/ω0∼10-8 s here, with the Larmor frequency ω0 of 100 MHz in the present measurements. The temperature dependence of T1 reveals that the activation energy in the SmCA* phase is larger than that in SmA for most of the carbon nuclei of the aromatic core part. It suggests that the molecular rotation is hindered in the SmCA* phase, in which the large spontaneous polarization exists in a layer.

13C NMR lineshapes for the 13C2H2H' isotopomeric spin grouping.

Carbon-13 NMR lineshape calculations applicable for deuterated methylene spin groupings are described. The interplay between anisotropic interaction strengths, isotropic couplings, and dynamic parameters are considered in this investigation. It is shown that 13C lineshapes for this system are informative and provide details on numerous factors that are difficult to ascertain by alternative means. It is demonstrated that in the vicinity of the deuteron T1 minimum, quadrupole-quadrupole cross-correlation revealed through the 13C lineshape, may provide a useful methodology for the study of molecular reorientational anisotropy in condensed phases. Similarly, quadrupole--dipolar interferences induce significant second order frequency shifts that provide dynamic information not accessible by means of conventional spin relaxation experiments.

Dynamics of anions and cations in hydrogensulfides of alkali metals (NaHS, KHS, RbHS): A proton nuclear magnetic resonance study

Second moments (M2) and spin-lattice relaxation times (T1) of proton nuclear magnetic resonance (NMR) measurements were analyzed for the hydrogensulfides of sodium, potassium, and rubidium. Three modifications are known at ambient pressure for each of MHS with M=Na, K,Rb. They are isotypic for the different cations, with an ordered monoclinic low temperature modification, a dynamically disordered rhombohedral middle temperature modification (MTM), and a cubic high temperature modification (HTM). The number of proton sites for one proton increases from two in the MTM to at least eight in the HTM. The title compounds were investigated by proton NMR in the temperature range 180 K⩽T⩽560 K with a resonance frequency of ν0=400 MHz. In addition, KHS was measured from T=90 K up to T=297 K, using proton resonance frequencies ν0=45 MHz and ν0=96 MHz. The M2 calculated from the absorption signal of the individual modification for the various cations do not differ from those calculated for the known crystal structures. The minima of T1 are in good agreement with results of calculations based on the crystal structure of the MTM of these compounds and two-site 180° reorientations of the anions. Activation enthalpies and attempt frequencies (NaHS:26.2(4) kJ mol−1, 6.1(2)×1014 s−1, KHS:19.0(4) kJ mol−1, 4.0(7)×1013 s−1, RbHS:16.3(4) kJ mol−1, 2.0(2)×1013 s−1) agree with those determined by quasielastic neutron scattering [Haarmann et al., J. Chem. Phys. 113, 8161 (2000)]. For the cubic HTM, translational motion of the cations is probable. Only in the case of KHS and RbHS, was the onset of this translational motion already observed in the MTM at temperatures close to the phase transition into the HTM. This seems to be a difference in the mechanism of the MTM⇄HTM phase transition for NaHS, on the one hand, and KHS, RbHS, on the other hand.

NMR Study of Phase Transitions in New Ferroelectric Crystal—(C5H5NH)5Bi2Br11

A T1 minimum at 216 K for Larmor frequency 90 MHz has been detected and for this minimum no analogous T1 minimum according to the known quadratic dependence of the Larmor frequency 25 MHz is found. The analysis leads to the conclusion that this T1 minimum is a result of the relaxation of protons via quadrupole nuclei. The Kimmich theoretical treatment of 1H NMR experiments exhibiting the existence of this phenomenon in the case of relaxation of protons of piridinium cations in (C5H5NH)5Bi2Br11 and the estimated averaged quadrupole frequency of interacting quadrupole nuclei has been estimated to be around 71 MHz. Below the phase transition at 118 K a wide symmetric spin-lattice relaxation minimum at 25 MHz is detected and a model of small angle libration of the pyridinium cation has been applied to explain the observed T1 relaxation time minimum.

Ammonium Tunnel Levels and Spin-Rotational Wavefunctions in (NH4)2S2O8

Proton spin–lattice relaxation in a single crystal of (NH4)2S2O8 was studied as a function of resonance frequency at various constant temperatures between 4.2 and 30 K. Two T1 minima were found, one at 8.6 MHz and the other at 4.3 MHz. They are related to the splitting between the lowest T level and the A level, equal to 8.6 MHz nearly independently of temperature below 25 K. Together with the large tunnel splitting of 269 MHz, determined earlier by Clough et al. (Chem. Phys.152, 343 (1991)) our results define the spin-rotational wavefunction of the lowest T level very accurately, although those of the two higher T levels remain largely undetermined.