βRELATIONSHIPS BETWEEN STRUCTURE AND CARBONYL STRETCHING FREQUENCIES OF THE -LACTAM RING AND THE 7-ACYLAMINO GROUP IN 1-THIA-AND 1-OXA-CEPHEM NTIBIOTICS. IMPORTANCE OF THE BENDING ANGLE OF THE C(4)-N(5) BOND FROM PLANE C(6)-N(5)-C(8) IN THE β-LACTAM RING