Hi Colin,
I understood the jest, of course.
Now I got curious: At 2theta=0, the scattering from H is 1e, so I assume
the the scattering length for its nucleus is 1e/1860 = 0.00054.
According to the data from Phil Coppens web site, the atomic scattering
factor for H reaches this value at sin theta / lambda = 1.95, i.e.
d=0.26A. That's far away from the wavelength 'we' use, but not too far
off from the resolution limit on a Silver source (0.31A), is it?
I am not sure this can be totally neglected. Am I wrong?
Cheers,
Tim
On 02/03/2015 04:03 PM, Colin Nave wrote:
> Hi Tim
> Although my SHELX comment was in jest, your point illustrates the programs versatility. You are also right about the flat(ish) form factor for the proton.
> To get to a resolution where there is a cross over would require a very short wavelength. Other processes would then dominate. A nice source for this is the x-ray data booklet from LBL, in particular the chapter on scattering of x-rays from electrons and atoms.
> http://xdb.lbl.gov/Section3/Sec_3-1.html
> Interestingly fig 3-1 in this does not include coherent scattering from nuclei presumably because it is negligible compared with the other processes - in practice Ian was correct in saying that a proton is effectively invisible to x-rays of the energy we usually use.
>
> Colin
>
>
> -----Original Message-----
> From: Tim Gruene [mailto:[log in to unmask]]
> Sent: 02 February 2015 22:08
> To: Nave, Colin (DLSLtd,RAL,LSCI); ccp4bb
> Subject: Re: [ccp4bb] proton scattering by X-rays
>
> Hi Colin,
>
> you can add f' for every atom type in SHELXL yourself, so in that sense, it has been incorporated in SHELX. Bear in mind that the nucleus is point-like to X-rays at ordinary wavelengths so that it should not have a form factor like the electron cloud but a constant scattering length - just as they do for neutron scattering.
>
> You can do the maths at what resolution the form factor and the constant
> 1:1860 scattering length contribution cross. It is not ridiculously small but nowhere near 0.8A. Charge density people may need to take this into account, but I don't know if they do.
>
> Cheers,
> Tim
>
> On 02/02/2015 04:03 PM, Colin Nave wrote:
>> “As you say the proton itself is invisible to X-rays.”
>> Not quite! The ratio of scattering between electrons and protons should go as the inverse square of the masses.
>> Ratio of mass 1:1860, ratio of scattering 1:3459600. A small correction but doubtless it has been incorporated in to SHELX.
>> Colin
>>
>>
>> From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of
>> Ian Tickle
>> Sent: 02 February 2015 13:35
>> To: ccp4bb
>> Subject: Re: [ccp4bb] proton scattering by X-rays
>>
>>
>> Peter, if it's a covalently-bonded H atom it surely can't be a bare proton, it must have at least some partial electron around it for the (possibly partial) covalent bond, enough to diffract X-rays anyway. As you say the proton itself is invisible to X-rays.
>> Cheers
>> -- Ian
>>
>> On 2 February 2015 at 13:08, Peter Moody <[log in to unmask]<mailto:[log in to unmask]>> wrote:
>> Dear BB
>>
>> I have (again) realised how limited by understanding of our subject is.
>>
>> In Nature’s online site http://www.nature.com/nature/journal/vaop/ncurrent/full/nature14110.html?WT.ec_id=NATURE-20150129 there is a paper describing an X-ray structure determined with sub-atomic data (nice!). The figures show density for H+ as well as H-. In my simple way I had assumed that any X-ray scattering from the nucleus was negligible, and that the electrons are responsible for this. I would expect a proton (i.e. H+) alone to be invisible to X-rays, and certainly not to look similar to a hydride (with two electrons in (electron density) maps. What have I missed? Could someone please explain, or point me to a suitable reference?
>>
>> Best wishes, Peter
>> (please use [log in to unmask]<mailto:[log in to unmask]> to reply
>> directly)
>>
>> http://www2.le.ac.uk/departments/biochemistry/staff/moody
>>
>>
>>
>
> --
> Dr Tim Gruene
> Institut fuer anorganische Chemie
> Tammannstr. 4
> D-37077 Goettingen
>
> GPG Key ID = A46BEE1A
>
>
--
Dr Tim Gruene
Institut fuer anorganische Chemie
Tammannstr. 4
D-37077 Goettingen
GPG Key ID = A46BEE1A
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