第1楼2004/12/22
N-15的谱是比较难做,因为丰度低,而且弛豫时间长。所以得用高浓度的样品,弛豫延迟最好长一些(如3秒),脉冲角度不要太大(如45度)。累加时间可能会较长。关于化学位移标尺,在蛋白质里是以液氨为0ppm,所以酰胺N在120ppm附近。而在小分子里好象用CH3NO2为0ppm更普遍,这时FORMAMIDE为-267。8ppm。以下是从一片文献里(Monatshefte fur Chemie 131, 437±449 (2000))拷来的作为参考:
Experimental
The 15N NMR spectra were measured using a GE 300MHz NMR spectrometer with an observation frequency of 30.45MHz for 15N. The non-decoupled 15N NMR spectra were calibrated against external formamide (ˇ267.8 ppm) and are referenced to the delta () scale with nitromethane set to zero [13,14]. Previous standardizations used liquid ammonia (ˇ381.9 ppm) [14]. Following the general practice, chemical shift values that are more shielded than the reference are given negative
signs and those less shelded will have a positive sign. This means that most 15N values are negative.Sample tube diameter: 10 mm; temperature: ambient; spinning condition: 15 Hz; RF pulse length:
10 ms (45). The addition of the paramagnetic reagent ferric chloride hexahydrate (FeCl3 6H2O,20 mg) helped to shorten the acquisition times for several compounds because it decreased the
Nuclear Overhauser Effect (NOE) [6].
Most tetraalkylammonium salts used in this work [1,7,9,12] were readily soluble in water.Aqueous solutions of 3.0M and above for the tetra-n-alkylammonium halide salts were not uncommon. Solution preparation in aqueous and methanolic reagents yielded similar results for obtained chemical shift data of these salts. The acquisition times of these solutions were about 1 h. N-Alkyl substituents on the nitrogen atom exerted the most important effect on the chemical shift of amines and ammonium salts. Other factors showed were of less inØuence on either the acquisition times and/or on the chemical shift values.