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hao_jian木虫 (正式写手)
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【求助】缺高角度数据怎么办啊已有2人参与
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我们的晶胞太大，经常因为晶体太小，导致高角度点太弱，有个A类错误没法解决，谁有高招，不胜感激：错误是： THETM01_ALERT_3_A The value of sine(theta_max)/wavelength is less than 0.550 Calculated sin(theta_max)/wavelength = 0.4999 测的晶体都不大，上机器测的都是好不容易找到的最大的单晶，但还不够大。 增加衍射强度的方法也试过n次，要么背景太强，要么信号太弱。 我想知道有没有什么后期处理的方式，把这个错误的影响降低，例如变成B类错误？ 按我们的经验，解的过程顺，无需大量限制键长和温度因子的数据都不错。根据这个经验，上述数据解出来的结构可信度非常好，由数据自然得到的键长和热运动都非常符合常规，而无需增加额外的不自然的限制。 我们测的都是非常大的晶胞，R因子、Q峰、温度因子等等要求都不是很高。 cif网站介绍此错误： PROC-NAME: THETM01 Type_3 PURPOSE: To check that _diffrn_reflns_theta_max is greater than expected limits. PROCEDURE: CALCULATE S = SIN [_diffrn_reflns_theta_max] / _diffrn_radiation_wavelength TEST IF S < 0.55 issue ALERT A "Alert A The value of sine(theta_max)/wavelength is less than 0.550" < 0.575 issue ALERT B "Alert B The value of sine(theta_max)/wavelength is less than 0.575" < 0.59 issue ALERT C "Alert C The value of sine(theta_max)/wavelength is less than 0.590" The Commission on Journals has determined that setting the maximum sin(theta)/lambda for measurements to at least 0.6 is both optimal and achievable for most structural studies. The extent of the data not only affects the ratio of measurements to refined parameters and hence the accuracy of the atomic and geometric parameters, but also the reliability of the angular dependent terms, such as Uijs. It is unlikely that any submitted CIF which contains the results of a data collection with a lower theta(max) will be accepted for publication unless there are exceptional circumstances. Old data claimed to have been collected when less stringent criteria were in place will not be excepted. You should routinely set your diffractometer so that a theta(max) of 25 deg. for Mo radiation or 67 deg. for Cu radiation is the minimum that is achieved. If the crystal diffracts sufficiently well, it is strongly recommended that theta(max) is set to an even higher value. It is understood that there may be little point in collecting background "noise" when a weakly diffracting crystal gives no detectable diffraction above theta values well below our required limit. In such a case, you should first ensure that you have conscientiously undertaken all possible steps to ensure that the experiment has been able to extract the best diffracting power from your sample. To this end, all of the following options should have been exhausted: (a) The best possible crystal has been sought and utilised for the data collection. If the crystal was small or of poor quality, are you sure that a better or bigger one cannot be obtained? The chosen crystal may have been the best one in the batch, but had enough effort and patience really been expended in the crystallisation attempt? Often, lack of experience, impatience or unwillingness to try another crystallisation technique or solvent leads to the experimentalist abandoning crystallisation attempts before the optimal conditions have been found. Before you blame the crystal quality, be sure that you have convinced yourself that nothing further can be done in this direction. (b) Use low temperature measurements to enhance the reflection intensities and the extent of the "observed" data. (c) For organic compounds, try data collection with Cu radiation, which significantly enhances the reflection intensities. [ Last edited by hao_jian on 2010-4-22 at 13:31 ] |
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铁杆木虫 (著名写手)
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2楼2010-04-22 11:33:13
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帝夫:谢谢回帖交流 2010-04-24 14:36
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谢谢您的意见！ 晶体都不大，上机器测的都是好不容易找到的最大的单晶，但还不够大。 增加衍射强度的方法也试过n次，要么背景太强，要么信号太弱。 我想知道有没有什么后期处理的方式，把这个错误的影响降低，例如变成B类错误？ 按我们的经验，解的过程顺，无需大量限制键长和温度因子的数据都不错。根据这个经验，上述数据解出来的结构可信度非常好，由数据自然得到的键长和热运动都非常符合常规，而无需增加额外的不自然的限制。 我们测的都是非常大的晶胞，R因子、Q峰、温度因子等等要求都不是很高。 【这一段话已经编辑加入1楼】 [ Last edited by hao_jian on 2010-4-22 at 13:38 ] |
3楼2010-04-22 13:19:00