menucheckCIF procedure
PROC-NAME: THETM01 Type_3PURPOSE: 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"
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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. |
