We present a 14N nuclear magnetic resonance study of a single crystal of CuBr4(C5H12N)2 (BPCB) consisting of weakly coupled spin-1/2 Heisenberg antiferromagnetic ladders. Treating ladders in the gapless phase as Luttinger liquids, we are able to fully account for (i) the magnetic field dependence of the nuclear spin-lattice relaxation rate T1(-1) at 250 mK and for (ii) the phase transition to a 3D ordered phase occurring below 110 mK due to weak interladder exchange coupling. BPCB is thus an excellent model system where the possibility to control Luttinger liquid parameters in a continuous manner is demonstrated and the Luttinger liquid model tested in detail over the whole fermion band.

By means of nuclear spin-lattice relaxation rate T(1)(-1), we follow the spin dynamics as a function of the applied magnetic field in two gapped quasi-one-dimensional quantum antiferromagnets: the anisotropic spin-chain system NiCl(2)-4SC(NH(2))(2) and the spin-ladder system (C(5)H(12)N)(2)CuBr(4). In both systems, spin excitations are confirmed to evolve from magnons in the gapped state to spinons in the gapless Tomonaga-Luttinger-liquid state. In between, T(1)(-1) exhibits a pronounced, continuous variation, which is shown to scale in accordance with quantum criticality. We extract the critical exponent for T(1)(-1), compare it to the theory, and show that this behavior is identical in both studied systems, thus demonstrating the universality of quantum-critical behavior.

Table VI. Non-Hydrogen Atomic Coordinates for 2c with Esd's in Parentheses y_z_B^, A2 Pt 0.23633 (4) 0.81275 (3) 0.171561 (20) 1.955 ( PI 0.3093 (3) 0.64987 (20) 0.20193 (13) 2.22 (1 P2 0.1980 (3) 0.76938 (19) 0.05211 (13) 1.93 (1 Cll 0.1430 (3) 0.98185 (20) 0.13735 (14) 3.74 (l• C12 0.2677 (4) 0.85678 (23) 0.29793 (14) 4.52 (1 Cl 0.3006 (10) 0.5791 (7) 0.1137 (4) L9 (4) C2 0.3570 (12) 0.4794 (8) 0.1127 (5) 2.9 (5) C3 0.3593 (12) 0.4323 (8) 0.0485 (5) 3.3 (6) C4 0.2986 (12) 0.4828 (8) -0.0194 (5) 3.3 (5) C5 0.2432 (11) 0.5819 (7) -0.0200 (5) 17 (5) C6 0.2462 (10) 0.6328 (7) 0.0479 (4) 1.9 (4) Cll 0.5006 (11) 0.6271 (7) 0.2583 (5) 2.7 (4) C12 0.5253 (13) 0.6568 (9) 0.3406 (5) 4.1 (6) C13 0.6126 (12) 0.6795 (10) 0.2239 (6) 4.7 (6) C14 0.1866 (11) 0.5835 (8) 0.2495 (5) 2.8 (5) C15 0.2235 (14) 0.4687 (9) 0.2670 (6) 4.7 (7) C16 0.0279 (13) 0.5950 (10) 0.2082 (7) 5.5 (7) C21 0.3156 (11) 0.8302 (7) -0.0006 (5) 2.5 (5) C22 0.2753 (13) 0.9441 (8) -0.0215 (6) 4.3 (6) C23 0.4767 (12) 0.8188 (9) 0.0415 (5) 4.1 (6) C24 0.0070 (12) 0.7950 (8) -0.0032 (5) 3.2(5) C25 -0.1047 (12) 0.7665 (9) 0.0390 (6) 4.7 (6) C26 -0.0320 (13) 0.7488 (9) -0.0824 (6) 4.5 (6)done with a Broker built-in panic program. The J values are according to the data in Table I, and LW = 1.6 Hz. All NMR data are summarized in Table I.All of the reactions, manipulations, and purification steps involving phosphines were performed under a dry nitrogen or argon atmosphere.Other chemicals and solvents from commercial sources were used without further purification, except as noted.Syntheses. o-Phenylenebis(diisopropylphosphine) (l)2c and dichloro-[o-phenylenebis(diisopropylphosphine)]nickel(II) (2a) were prepared according to the literature methods.3dDichloro(o-phenylenebis(diisopropylphosphine)]paUadium(II) (2b). A mixture of DH (0.0723 mmol) and (PhCN)2PdCl2 (0.0723 mmol) in degassed dichloromethane (4 mL) was stirred at room temperature for 5 h, during which the solution became brown. Methanol (3 mL) was added to the reaction mixture, and the resulting solution was allowed to sit overnight. A colorless crystalline solid was collected (27.6 mg, 78%; mp 275 °C dec). Anal, Caled for Cl8H32P2Cl2Pd; C, 44.33; , 6.61.Found: C, 44.35; , 7.00.Dichloro[o-phenylenebis(diisopropylphosphine)]platinum(II) (2c). A flask was charged with DH (0.59 mmol) and K2PtCl4 (0.59 mmol), followed by a mixture of degassed water (5 mL) and ethanol (5 mL). The resulting suspension was stirred for 3 h, after which white precipitates were collected by filtration. The filtrate was extracted with CH2C12 to recover the remaining product, and the combined product was recrystallized from CH2Cl2/methanol. A colorless crystalline solid was obtained (195.2 mg, 57%; mp 280-285 °C dec). Anal. Caled for

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